And I'll go with DPS. Assuming everyone knows the mechanics, a bad DPS can fade into the background in trials. A bad tank or healer in trials gets everyone killed. A new and/or bad DPS is going to have to go out of their way to hurt the group.

Its the same for dungeons. If you are going with a Tank + 3 DPS setup, one bad DPS is not going to be as noticeable as a healer not healing or a tank screwing around.

Easy is relative.

I enjoy tanking because I don't like waiting for groups, I'm a bit of a control freak, and I don't mind telling other people what to do to succeed. It's not hard to execute the duties of the class per se, for example you needn't master some complicated rotation or min-max your stats to eek out every bit of performance. Instead, you need to learn encounters and use that knowledge to lead your team. If you're interested in learning and leading, it can be 'easy'.

Healing is also in the vein of the tank - though I would say requires less leadership and more encounter knowledge. You're responsible for keeping your group buffed and healed. Until you experience encounters, you won't have a great idea of where the damage spikes happen. Once you know encounters well, it becomes fairly painless to heal effectively through a lot of content - so some of the non-DLC vet dungeons can become pretty easy, assuming you have a remotely competent group. I enjoy healing content I know well, it's very 'relaxing' as opposed to DPS or Tanking because you're not worried about executing flawless rotations, nor leading the group - just preparing for damage spikes you know are coming and managing your team. More than any other role, as a healer you'll also be more aware of your group's strengths and weakensses.

IMHO DPS is the most difficult role to 'master' only because you'll constantly be pushing to maximize a flawless rotation, keep up all your buffs and pots and doing as much damage as possible while also (hopefully) trying to avoid mechanics and take care of yourself when healers and tanks are being thwarted by the game's mechanics themselves. That being said, unless you're in progression/score trials pushing for difficult top end stuff, you can be a suck dps and the 7 or so other dps in a normal or less challenging vet trial will compensate. You can certainly be a mediocre dps in the game with more success than a mediocre tank or healer.

Unless your tanking absolute endgame stuff like Vet AA DK Tanking is probably the easiest just sit there taunt boss every 15 seconds, chain adds, block heavy attacks and turn boss away from group and as long as you don't die to often nobody really cares what gear you wear,

then healer assuming group is competent

DPS is probably the hardest role in the game 1 bad DPS can mean the difference of a dungeon taking 30 mins, 2 hours, or not completing content at all most groups can get by with a decent tank and healer but dps more stressful most endgame groups require certain DPS numbers etc...