Stargate SG1 was one of the best science fiction television series ever, so it is very difficult to choose only ten episodes, however, after a very careful selection, these are the winners per season (Beware of spoilers!):

Season 1

Episode 19: There But For The Grace of God

SG1 arrive on an alien planet and estimates the level of technology is similar to Earth's, the place where the Stargate is seems to be some kind of base for off world explorations, however, no one is around. While Daniel is exploring in other room, Teal'c warns O'neill and Carter about a symbol left behind by Goa'uld conquerors, it means the surface of the planet is radioactive and they must leave immediately. Jack then yells, calling for Daniel to leave with the team but before that, Dr. Jackson finds an alien mirror made of naquadah, when he touches it, some kind of electricity goes through his body, he packs some things he finds around and goes to find the rest of SG1, only to find out they are gone. Daniel dials home but just as he arrives, the people at Stargate Command don't recognize him and imprison him. Later he discovers that he is in an alternate reality, and in there, Earth is under attack by the Goa'uld!

Why It's the best of Season 1. This episode depicts what would happen if the Goa'uld attacked Earth, it's an alternate reality but that doesn't make it less real. If Daniel hadn't experienced this, Earth would've been probably doomed.

Fan Bonus: We can see a Ha'tak landing on top of Cheyenne Mountain (We had only seen a glimpse of what a Ha'tak would look like in episode 14, Singularity). There is a kiss between two members of SG1.

There But for the Grace of God
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Season 2

Episode 15: The Fifth Race

The team gate to a planet and find themselves inside a stone dome with no exits, only a circular symbol on the floor, when Jack steps on it, an alien device materializes out of the wall, Teal'c looks at it and nothing happens, but when O'neill looks inside, the device traps his head for a few moments, then let's him go unconscious. When they return at Stargate Command, Jack shows no damage symptoms but as days go by, he starts developing a strange condition, his language is being rewritten by an alien one, not only that but he apparently also has all the knowledge of the alien species known as The Ancients.

Why It's the best of Season 2. It is revealed that the Goa'uld didn't build the Stargates, it was The Ancients. Earth now has a huge advantage, thousands of gate addresses unknown to the Goa'uld.

Fan Bonus: We get to see the Asgard one more time as they save the day.

The Fifth Race
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Season 3

Episode 15: Pretense

This episode begins with two ha'taks in the orbit of a planet trying to destroy a death glider, suddenly, several energy shots from the surface destroy both ships and make the glider land-crash. People hurry to see the glider and then we see who is inside, Skaara, injured and pleading for help. The planet is the new home of the Tollan, who summon SG1 to a trial in which it will be decided who is the owner of Skaara's body, himself or the goa'uld Klorel. During the trial, the goa'uld plot a plan to destroy the Tollan

Why It's the best of Season 3. The ultra advanced alien race known as the Tollan returns to kick the goa'uld in the butt, and in the process, to be saved by SG-1.

Fan Bonus: The Nox make a new appearance.

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Season 4

Episode 6: Window of Opportunity

SG1 is on an abandoned planet with ancient ruins and a sun emitting huge coronal emissions. There is also an alien archaeologist who has been studying the planet for quite some tine. Out of nowhere he grabs a weapon and shoots at SG1, then, as he struggles with O'neill and Teal'c, activates an alien device that sends a lighting blast to the Stargate and after a white flash all disappear, then O'neill find himself eating fruit loops at Stargate Command, 10 hours in the past. Teal'c and O'neill are the only ones who realize they are back in time, but after a while they discover that they are, in fact, trapped in a time loop that repeats every 10 hours, when the alien device activates.

Why It's the best of Season 4. It's a hilarious episode, probably the funniest through all the seasons. It is also an homage to the movie Groundhog Day.

Fan Bonus: When Daniel tells Jack and Teal'c that they could do anything if they know everything is going to revert 10 hours later, they start doing all kinds of crazy stuff, including playing golf through an open Stargate.

Window of Opportunity
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Season 5

Episode 21: Meridian

This could be either the best or the worst episode of the season. It is very emotive, since it's the one where Daniel Jackson leaves SG1 (temporarily). The team is in a planet where the government is developing a naquadria (an element similar to naquadah but with huge energy emissions) powered nuclear device. When the negligence of the scientists makes the bomb unstable, Daniel sacrifices himself by receiving a deadly dose of radiation before switching the device off.

Why It's the best of Season 5. The series will never be the same, although Daniel Jackson is saved from a certain death by Oma Desala, he must leave, maybe never to return.

Fan Bonus: Not all is lost, Daniel doesn't die but ascend to a higher plane of existence, there's a possibility of return.

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Season 6

Episode 22: Full Circle

Anubis has a new powerful ship, and he goes to Abydos searching for a device that will make it even more deadly, SG1 must defend the planet against him, however, they are not alone, an old friend is back to help them, and if necessary, to destroy Anubis once and for all.

Why It's the best of Season 6. All the people of Abydos is killed off and then ascended by Oma Desala. Daniel Jackson fights Anubis using his new ascended powers, or at least he tries.

Fan Bonus: The return of Daniel Jackson! We also get to see the Abydos pyramid one last time.

Full Circle
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Season 7

Episodes 21 & 22: The Lost City (Parts 1 and 2)

It's a two parter, so it counts as one. SG1 finds a second Ancient repository but before they get the chance to analyze it, Anubis attacks the planet, O'neill uses the device again to save the knowledge and defend Earth against the Goa'uld. Later, Bra'tac arrives at the SGC and informs them that Anubis is gathering his fleet, he will arrive within three days. Will Jack be able to use the knowledge of the Ancients to save Earth? He does, he makes the team travel to an alien planet where there is an energy source that powers an Ancient defense system in Earth, crushing Anubis' fleet.

Why It's the best of Season 7. The final battle is truly epic, Goa'uld and Earth ships fighting with all they've got.

Fan Bonus: Prometheus shows up to save the day, commanding a lot of F-302's to fight the Goa'uld fleet.

Lost City: Part 1
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Season 8

Episodes 16 & 17: Reckoning (Parts 1 and 2)

The replicator threat in the Milky Way becomes so dire to the Goa'uld, who had been systematically attacked, that Baal asks Earth for help. There are rumors of a powerful weapon capable of destroying the replicators, the only problem is that it is in the Jaffa's holiest city of the goa'uld empire, deep within Baal's territory, Baal now serves Anubis, so the alliance must remain in secret. They manage to find the device and power it, simultaneously connecting the planet's Stargate to the whole galaxy's network, obliterating the replicators.

Why It's the best of Season 8. There is a space battle between the whole Goa'uld fleet (It's ultra huge) against an even bigger fleet of replicator controlled goa'uld ha'taks. This episode marks the end of the goa'uld hegemony in the Milky way, and also the destruction of the replicators.

Fan Bonus: As the Stargate connects with the entire galaxy's gate system, we can see familiar planets from past episodes. The action in the final battle is mind-blowing.

Season 9

Episode 15: Ethon

SG1 must rescue Daniel Jackson from a planet that is under the influence of the Ori, however, a prior has given the government of that world a weapon capable of destroying all their enemies, a satellite with enough firepower to destroy even the mightiest Prometheus.

Why It's the best of Season 9. We finally see what the Ori weapons can do.

Fan Bonus: Many people didn't like Prometheus, and now they can see how it is destroyed.

Season 10, We have a draw.

Episode 6: 200

Stargate SG1's 200th episode, a great accomplishment. If there is a funnier episode than Window of Opportunity and The Other Guys, this is it. The episode is full of jokes, parodies, homages, puppet characters, and more, they even shoot Mitchell (loved it).

Fan Bonus: We get to finally see the furlings, or at least what they may look like.

Episode 20: Unending

The Asgard summon SG1 aboard the Odyssey to a remote planet, they install Asgard shields and weapons on it and tell the team that their time has ended, the Asgard are about to die out, so they have installed an Asgard core computer that contains all of their knowledge. As many Ori ships are approaching, the Asgard beam away and destroy the planet, hoping that it will destroy the Ori ships, and also committing suicide in the process. The Ori ships catch up with the Odyssey and manage to overpower it, as they fire the last deadly beam, Carter is able to use the Asgard core to slow time. Now SG1 is trapped in the odyssey almost frozen in time, trying to figure out a way to escape, and it takes 50 years. 50 years living on the ship, sharing their entire lives together. After that time, Carter finally finds the solution, they use the enemy beam as a power souce to reverse time and send Teal'c moments before the Ori destroys the ship.

Why It's the best of Season 10. It's an epic end for an epic series. The end of an era.

Fan Bonus: We get to see how SG1 (minus O'neill) live their lives together for 50 years.

OK, there is more that 10 episodes, but it was truly impossible to reduce the number, there are so many great episodes. ¿What are your favorites?