Which MMO is the hardest you've ever encountered. I mean in terms of learning curve, the difficulty, and the menu navigation. I'm actually looking for one so I can try it and see for myself.
For me it was Tera just because the combat was different from any other MMO I have played, but it wasn't hard it took about an hour or so to adjust.
With my brief excursion into EVE online, I have to say that I've never felt so overwhelmed in my life. It was incredibly daunting to get used to all of the little details and find my way around the GUI and space. I got the hang of it after a week, but by that time my trial had ended and my interest had faded.
Eve Online is only technically the hardest MMORPG because it has the absolute highest learning curve of any game out there. That is not difficulty though. Once you learn the mechanics the game can be relatively easy. Difficulty is about the game being challenging for all players, not just the new ones unfamiliar with the mechanics. For me I would say it's a toss up between WoW and Guild Wars 2. Both of them have varying levels of difficulty depending on where you are and what you do, but the group based content such as the raids and dungeons can be an absolute nightmare without proper organization and gearing.
Definitely Vanilla WoW here. Some of the most difficult gameplay I've ever experienced (in any game) was in Naxx, BWL, and AQ40. The simplicity of the game required a serious know-how to things and made sure that you were always on top of your game. You had to be constantly aware of what was around you as well as perform well. If you could do this you would beat many fights - considering that the rest of the raid did the same.
I feel that no game is truly the hardest. Most games are just mindless grinds and for a casual player it's really all the same. For me the hardest thing about any game is finding enough time to dedicate myself to it and get to the highest level in order for the game to actually be hard. I think any game that can do this and get you hooked on long enough to make the game hard is hard.
Ultima Online in it's first 2 years was really tough. Not the gameplay itself per se but rather the fact that PvP was open in anywhere but big cities. Your chance to get PK'ed was nearly 99% unless you knew all ins and outs of fighting and your char was properly trained for that. So all the best mob hunting spots had tons of reds and other PKs and blue anti-PKs fighting all over the place.
I would have to say Tibia. I played it coming from a background of not-too-hard MMOs which were comparatively casual for what was waiting for me there. I tried it out due to curiosity and my friend who's a big fan(and also a very good gamer in all games) pushing me. I failed so much, and just felt it, how that game was made in a completely different league, back when MMO's used to be something truly hardcore(and not just time consuming).
Seems like a lot of people are saying that EVE online is hard, I wonder how hard it actually is. What kind of game is EVE?
Oh I almost forgot to mention Runescape. When you die in that game you lose all of your inventory items except for your best three ones. That can be very brutal and unforgiving, but it makes the players treat it very seriously. I honestly wish more MMOs would do that. This may have changed recently though. I haven't played the new Runescape.
Old school Runescape was pretty brutal, but that's more because of the death penalty that they used to have. As of now, I heard Tera is pretty challenging. Lord of the Rings online is also worth mentioning.
The new Runescape is very hard for me to play. I remember playing in the old school era when everything was so simple (in terms of quests, interface, and just simplicity in all aspects of the game in general). However, they've added so many new interfaces to the game that it's actually harder for me to see where i'm actually going than do anything fun.
I'm going to echo a bunch of people here and agree with EVE as being one of the hardest MMOs out there. You can pick it up and muck about sort of casually, but you won't really ever get anywhere. To truly excel you not only need enormous amounts of out of game study time, but also constant dedicated in-game work.