I have been working on the Best In Slot list for my Witch Doctor in Diablo 3. I am actually remarkably close to it now as I am only missing the weapon - and what a weapon it is. (Starmetal Kukri) Because every piece has a ton of random rolls there will always be room for improvement but the improvements are going to be pretty minimal at this point. One thing I really like about the gearing in the D3 expansion is that the lists of optimal gear are really mixed up. Different items have such interesting and variable abilities and when that is combined with the randomness of loot due to limited trading there are a million and one different builds and setups.
One extreme outlier is the Ring Of Royal Grandeur. RoRG has a secondary ability that makes you count as if you have one more of every set you have equipped as long as you have at least two of that set. For example, I can wear two pieces of the Aughild's set and get the bonus for having three pieces. This would be a great ability even if it only applied to a single set but it applies to all sets and as such this ring is Best In Slot for everybody. I am currently using it to complete three different sets and the list of things this ring is doing for me is completely hilarious.
15% reduced damage from elites
15% more damage to elites (Aughild's 3p)
Immune Molten, Desecrator, Plagued (Blackthorne's 4p)
Gain 8 permanent dorks that are a brutal beating (Zunimassa's 4p)
Any one of those things would be an excellent and sought after bonus, all of them together is bonkers.
Not only is RoRG insane for endgame gear compositions but it is off the rails for gearing up too. I only own three Zunimassa's pieces right now and the four piece bonus is ludicrous. RoRG also lets you muck with slots like crazy because you can decide which slots of a set to use. For example, I have a crazy good hat and RoRG lets me get my set bonus without putting a drastically inferior set piece into the hat slot. RoRG gives you incredible power when gearing up, amazing flexibility, and is undoubtedly the best item in the game.
The weird thing about RoRG is how you go about getting it. Most items in the game you simply have to kill monsters and hope, though some you can gamble for. RoRG is found by completing a particular series of quests (full act 1 bounty runs, in particular) and then hoping that the 2% chance of a drop comes home. Some people get lucky of course, as Spots managed to get a full five copies of RoRG, one for each finger, before I got my first one. I was somewhat unlucky as it took me about ~170 tries to get my 2% to work but of course that isn't particularly outrageous though it is incredibly frustrating watching other people get them quickly and effortlessly.
After a couple of days of runs I was starting to get pretty tired of opening ten boxes every forty minutes and getting nothing from them. When randomly killing dudes it is a very different feeling because you have no idea what exactly you are farming for but when there is a singular item you want to find it feels terrible to grind away at that item for a long time and fail.
Overall I think it is pretty cool that there are a bunch of items only available from specific sources and there are all kinds of crazy things to farm. This feels like a fantastic, wonderful loot system. Maybe it is fine to have a single item that everyone should wear but I don't like it personally - having items that are amazing is all good, but I think they shouldn't be amazing for *everyone* but rather should be amazing for some people and not others. Still, the loot in the D3 expansion is so well done in general that I cannot possibly fault the whole. This fussing about RoRG is just the perfectionist in me nitpicking.