WoW is the most played MMO, and is pretty casual these days, and has a lot to allow players with not a lot of time to still see a fair amount of content. It isn't quite as much of a time sink as it was.
Elder Scrolls Online is a good game, more story driven than WoW, and I think a fair amount of people still play it. Also, I think if you like the Elder Scroll franchise, you will probably enjoy it a little more.
Star Wars:The Old Republic has dwindling numbers, and it the most single player game that is an MMO, imo. However, the story is pretty good and if you are a Star Wars fan, pew-pewing with the blasters, and schwoom-schwooming things with light sabers is pretty fun. Went free to play and a lot of kind of necessary things are now behind a paywall. Subbing gets you these things, and you can still pay real money to get items, etc.
Guild Wars 2 is a very pretty game, with a fair amount of people still playing it. Lots of customization of your characters look, and some really cool ways of handling crafting, bank storage etc. Added bonus is no sub fee.
Rift is free to play, which means it has a lot of stuff you can (only) get in the shop with real money. I never made it to the end game stuff, but it is also very pretty game, has the one of the best player housing set ups ever, and is fun.
Finally, Wildstar, which i don't think too many people play anymore, and will soon be going free to play, but like SWTOR, will give you 'extras' if you want to sub. Most cartoony looking MMO i have played, but the dialogue and overall feel is way more snarky and funny. Has the second best player housing setup, with a lot of customization you can do to your own personal "space".
Also, i googled and found this, which has a lot of the games I mentioned:
http://massivelyop.com/2015/06/11/perfect-ten-the-top-10-healthiest-live-mmos/