First of all this is hard to define as there are so many different kinds of online RPGs. As far as text based I will of course be biased and say the ones here are the best compared with other text based ones I have encountered. For graphic based RPGs I used to love Blizzard based games, but I am not involved in these anymore.
Like, MMORPG's? Um...Probably Warhammer:Age of Reckoning I actually capped a character when it first came out. The PvP is brutal, and satisfying as compared to other mmo's. I played WoW for a little while after I had played Warhammer, and I completely demolished all competition in PvP. Honestly, I was probably average in Warhammer, but I had to work to get that way. Not to mention public quests are awesome. My favorite moment came when I was running down the road, then I found a beach being besieged by orcs. I joined in the fray, and after a massive battle, aided by random players, I killed the orc chief guy and saved the beach, earning a Rare Item for my efforts.