Right now that would obviously be Dungeons & Delvers: Black Book, because I specifically designed it to do everything I wanted it to do (which is what I assume is the goal of most game designers), but who knows there might be things that crop up in the future that I'll want to add/change/remove.
Otherwise there aren't any RPGs that I just use as-is, because every RPG I've played has one or more flaws that I'll either just put up with or change. This is what eventually caused me to make my own game.
For example, I enjoy playing 4th Edition Dungeons & Dragons, but I ended up doing the half-HP tweak, making my own skill challenge system because the examples in the book were just terrible, and letting players make their own powers and even "Essentialize" classes to make things simpler.
In Dungeon World, for starters, I scrapped the entire Undertake a Perilous Journey move (since nothing about it makes any sense at all), did a post on how I'd change the bag of books, dunno how I'd change Hack & Slash and Backstab to account for the in-book contradiction (attacking an unaware monster is supposed to be an auto-kill, but Backstab just does more stuff), and Melissa and I have almost made a variant for every core class.
Also need to bump up the dragon's HP, since by the book it should have 20 and not 16.
After putting it to a vote, the next couple of classes on the docket are the warden (think 4E D&D warden) and apothecary (gotta go see what they're all about).
Dwarven Vault is our sixth 10+ Treasures volume. If you're interested in thirty dwarven magic items (including an eye that lets you shoot lasers) and nearly a dozen new bits of dungeon gear, check it out!
Just released our second adventure for A Sundered World, The Golden Spiral. If a snail-themed dungeon crawl is your oddly-specific thing, check it out!
By fan demand, we've mashed all of our 10+ Treasure volumes into one big magic item book, making it cheaper and more convenient to buy in print (which you can now do).