Well, I've only really played two MMORPGs, So I can only give my opinion on those.
I play WoW myself. Interesting gameplay + extra content is most of the patches keeps me playing, not to mention that I've made a lot of good friends there. The only thing is, if you're a casual gamer and only spend a few hours a day on it, you'll quickly be out-leveled by most people online (unless you get powerleveled, but what's the fun of that?)
Wulfur wrote: I personally play WOW, but unless you get on a new server or ones with some good friends you will be mostly solo playing till you get to the BC content which would be getting to lvl 58. most people that are playing are trying to beat all the end game stuff so it is hard to find people to lvl with at the moment.
I've never had that problem to be honest

Might just be my server, but there are plenty of lower levels running around. And not all of them are Draenei or Blood Elves either.
Guild Wars.... Well, I'm going to have to say I'm against this one. Admitedly I haven't played it sicne it first came out, but I had a lot of problems with it. People were generally unfriendly, and I was only able to talk to them in towns, once I got out into the wilderness I was by myself... Which makes for good solo play, but not really what you want in an MMO. Also, one of the patches caused my computer to have a seizure and take permanent damage. A lot of things were deleted, and my Graphics card died: it could barely handle NWN, and even then it was really choppy. I know this isn't the case with every computer, otherwise GW wouldn't have any players. And it being free to play is definately a bonus.
Now that I think about it, I played a demo of SWG. And.... I wasn't very impressd with it either to tell you the truth. Although it had a lot more character customisation than WoW, it's gameplay leaves something to be desired. I'm not a fan of rapid clicking to shoot or attack in a RPG. Also the spawn rate was insane. Took only a couple of seconds for the enemies I faced to pop back up and shoot me, resulting in a couple of deaths.
But in the end, We're only giving you our opinions. The best way to choose which to play is to experience it for yourself. Here are the links for you to download the XX day trials for WoW and SWG. It'll let you play around with each game (for a couple of days anyway...) and see which you like best. You could probably find XX day trials for the other MMORPGs out there, but I'll let you find those since my internet isn't running well at the moment
WoW 10 day trial
SWG 14 day trial (Tab at the bottom)