quite a few mmos (paid ones) have 14 day free trials (usually with a level cap too) so you can just pick up what ever and give it a whirl.
only probably is I find to properly enjoy an MMO you need to be doing it with friends, either RL or one's that you make in game. running around by yourself can be boring as sin... but throw some friends in the mix and it takes some of the grind out of it
WoW - decent, the generic MMO to which pretty much anything MMORPG like is compared against. quite a lot to do, lots of end game content, pretty damn easy for new players as they've added billions of tool tips and hints.... basically hold your hand through the game xD
SWTOR - loved this when it came out, loved the story lines as they made you more involved over the genericness of wow. though lacked end game content so wasn't much to do at top level. it's now free to play I believe but with paid benefits from having a premium account? meh
ESO - like but never really got into the elder scrolls games... gave this a whirl. pretty fun to be fair! but after buying the game and playing my free month....decided not to sub. this was mainly due to our clan losing some interest in it and everyone was spread out through the levels so getting groups was a pain as would either be super easy for some or silly hard.
Runescape - oh lord....lol....free to play, simple graphics.... not really sure as not played it much (like 5 mins on an ex's brothers account lol ) but seems to be made up of billions of kids.
EvE - pretty cool space mmo but a brutal learning curve and will take a while to work towards anything you want to due because of its skill learning system. but it is pretty fun

but the most expensive to sub for.... unless you're pretty good... then you can make enough ingame money and use that to pay for the sub... which I think all mmos should have xD
there are soooooooo many MMOs out there it is unreal, tera, aion, knight online etc etc etc etc etc.... see if any of your mates play some or are interested and just see what takes your fancy and give it a go
I've just returned to Ultima Online after about 8 years xD back in the day it was amazing, great community, well populated world, cheaper

but now it's 9.99 a month (thanks EA.....), and unfortunately... not so well populated (like.... barely see others out and about

) this was my first MMO I played and had the first guild/gaming community I joined

lots of fond memories.... but as I said..... a big part of it I feel is who you play with.... and now all my guild aren't active (last was last month... before that... 2012

) and the decreased population on the server it does feel a bit lonely
