I plan to do most content alone, but I will do some grouping. I just want be able to play alone when I want. I'll try to convince my friends to play if I love it. So which is sufficient?

All of them.

PvE is pretty easy as long as you stay around the levels it tells you to.

The world bosses are quite harder though, you'll need a gang of people for that.

I'm surprised noone said "Guardian and Elementalist" then walked out the door.

I agree with the above posters - all of them are if you play intelligently, though I'd say the classes with higher health pools or more self healing (warrior, guardian, necromancer, ele) generally give you an easier time of it. Oh, and there's Mesmer, but they survive differently than other classes.

Pretty much this. I've spent a good amount of time with a Necromancer and Elementalist (level 80 and 60 respectively), but also found Guardian, Engineer and Ranger pretty good with the small amount of time I tried with them.

Guardian. Bar none, Guardian. It's facerollingly easy. Because free passives are AWESOME.

Guardian and elementalist.

/walksout

/walksbackin

Also necro, and warrior to a certain extent. As has been said, though, as long as you toss some points into vitality and toughness (and play smart), anything in PvE besides dungeons won't present too much trouble for you.

Guardian because of all those passive health buffs you get. But you can solo the content with any of the classes.

Just play each until lvl20 or so to find what you have fun playing, they're all rather balanced for doing stuff alone or jumping in groups. Really depends alot on if you like ranged or melee & go from there. For me I found that I preferred the necromancer, mesmer, thief & guardian the most out of all the classes.

I wouldn't say that, so much as "what archetype do you enjoy?"Every profession - save the engineer - has viable options for range and melee. The biggest variations are maximum and minimum distance for ranged weapons in particular.