Diablo 3 has been out for quite some time now, and patches have constantly changed the best way to gear up your character. Today before 1.0.8 patch hits things aren't going to change much, but times are sure to be different even after this writing. With that being said, here are some of the most efficient ways to build your character's gear and be competitive in multiplayer games and solo farming.
Figure out what you want
Before going into the gear choices and selections you must determine what kind of player you are and what interests you. Some people like myself love to farm MP1 and get as much experience as we can get, so offense is more preferred than defense. If you are looking to farm higher MP levels to do key runs or guarantee better boss drops then you are going to have to opt for more defense to survive being hit by more powerful enemies. The other thing that must be considered when gearing up in Diablo 3 is your budget. Obviously we could put all this to the side if we could buy an 80 all resist perfect roll Witching Hour that has defense and offense, but then you'd need over a billion gold to buy that which 99% of the player base doesn't have. The fun of this game is getting gear, so work your way into your niche and go from there.
Obviously nobody wants to be a pure tank in Diablo 3; damage is a must. How much damage you decide to get for your character again depends on your style and what you plan on doing. The main forms of damage increase are critial strike damage and chance, attack speed, and your primary stat (i.e. Int for wizard, dex for monk, ect). There needs to be a good balance between the two, as having 60% crit is terrible if you have no crit damage to back it up. A good balance between defense and offense you are probably looking at about 80k damage, but there are people that are breaking 300k damage easily and are much more offense focused. Find out what balance you are looking to get and who you play with, as maybe there are more defense oriented players to work with so you can go all out offense to destroy bosses and farm easily.
The other thing to consider is defense. Again this game is so vast that I can't go over the specifics of each class and all the ways they have to gear, but having some all resistance and armor will aid in your farming so you don't die instantly when fighting a boss or die immediately when you get hit with a frozen affix from a champion pack. Utilize your class's passives and find ways to get the maximum benefit for your defenses so you can use more abilities and passives for offense and kill things quicker. Here's a general layout of what you should be aiming for in this day and age of Diablo 3.
Barbarian: Barbs play in a variety of ways, but most like the WW style so high lifesteal and damage is a must. You are going to be flying around the map AOE destroying huge packs of mobs so while offense is great to have, you will also need defense or they will stomp you right away. Most barb's prefer a high crit build with great lifesteal from Bloodthirst and their belt / weapon so survivability won't be as mandatory since you can steal back all the damage you do back as life. Aim for 100k while still keeping your vitality and armor high and you should be fine.
Monk: Monks are more defensive in nature, so again this is purely preference. Monks enjoy more attack speed in their typical set ups, though I have seen a bunch of crit monks out there as well. One with everything passive double dips into all resistance so you have the ability to get very offensive while still having defense since you can find a few pieces of gear with a lot of all resist and a single resistance stat that stacks so you can get some other pieces of pure offense gear like Inna's pants. Aim for around 70k with high all resistance to start and work from there, focusing on attack speed and crit chance as your dps boosters for sweeping wind and crippling wave.
Demon Hunter: Demon hunters have great survivability passives and abilities so they like to typically go glass cannon to crank out as much damage as possible. Shadow power with Gloom rune gives you tons of lifesteal and percent damage reduction for elites so you can still be glass cannon for regular mob packs and be tanky for elites when they come up. I see most average demon hungers out there easily breaking 100k damage with bare minimum defenses. I would say that having a little bit of armor and all resist would be helpful for when shadow power is down and you encounter reflect elites or a tricky situation, or you can just smoke screen and run away to continue to pound on them. Aim for 100k damage as a start and work from there to see if more damage is needed or some survivability. You probably won't need much survivability as a demon hunter but it depends on your play style.
Witch Doctor: The witch doctor is my personal favorite and my main class I play. Witch doctors are more strange to build because they can be both offensive and defensive, but can be quite squishy if not looked after properly. I prefer a hybrid approach to gearing because I use zombie bears as my primary damage ability so I'm often in the face of huge demon packs. My pets do a great job of tanking but if you opt out of a pet based build it's just you and you don't have as many tricks as a demon hunter does unless you pile up on CC like zombie walls and fears. I have 75k damage and pretty nice all resist and armor numbers so I'm not going to be hitting high MP's quite yet, but at the same time my damage is great for things like MP1 to plow through and farm the cash to buy better gear. Depending on your build, you could go much more offensive or choose a more defensive approach like myself. Attack speed isn't very good because of our mana issues, so I recommend a more crit based offense with high Int thrown in for good measure.
Wizard: The Wizard could change as Blizzard releases more patches, but right now wizards are unique in that they can go very glass cannon while still having great survivability because of critical mass. You can crit like crazy and reduce the cooldown of your survivability abilities so you can continue to burst dps and survive with diamond skin and teleport. Archon is getting a big boost in the 1.0.8 patch so many wizards are going to be ditching the CM build for an archon based build. Wizards are very squishy if you aren't geared to practice the CM build so because of this I would recommend that you start off with a fair amount of all resist and armor for defense and slowly build up your offense because you can't farm if you are dead. Have a good base defense for your character and as you get more gold buy gear that trades it off for big leaps in offense and you're golden.
I'm sure that a few months down the road this article will be dated and missing some key elements and while I will try to update it often I can't promise that. However unless things rapidly change and certain stats get nerfed or changed (Like attack speed back in the day) then I don't foresee this advice as being radically different for future patches. Having more offense is better, but some defense is needed so you aren't taking constant dirt naps. This is the best way to gear up in Diablo 3 and hopefully this helps you in taking out legions of demons.