The only games I've ever played that came close to UO were pen-and-paper Dungeons and Dragons and the Elder Scrolls series (Daggerfall, Morrowind, et cetera). What they all have in common is the open world. You're free to go where you want and do what you want no matter how long you've been playing or what your skill level is. If you want to go get yourself killed by a god or godlike monster as a newbie in any of those games, you can manage it. Now I haven't tried every other MMORPG out there but I've tried a handful of the more popular ones and I just didn't get the same feeling from them. They all seemed (in varying degrees) more linear, guiding me along a preset path instead of letting me carve my own.
I wanted Elder Scrolls Online to be legendary but I beta tested it and for various reasons I feel that it fell far short of its single-player predecessors. If I were prepared to fork over thousands of dollars all at once to play an MMORPG, I'd probably do Shroud of the Avatar. It seems pretty incredible but at the same time it's pay-to-be-awesome and I couldn't play as a peasant without constantly getting jealous of all the big houses and coolie stuff that I didn't buy.
tl;dr - My two cents is that UO is still the best online game and nothing else compares. SotA looks great but I'm not a huge fan of paying for internet, paying for subscription to a game, and then having to pay even more for all the cool/best stuff in the game.
