I'm not really a fan of MMOs myself. I've tried a few but can't get into them.
The one I really I have really liked has been Phantasy Star Online. It's definitely skilled-based and plays in some what like a dungeon crawler which is why I like it. While it might require some grinding, but the gameplay makes up for it.
I'm currently looking forward to Phantasy Star Online 2, which is going to be free-to-play. I don't know it's system requirements, but I know I'll need to buy a new desktop because my laptop really heats up while even playing Street Fighter X Mega Man. Or I could always get a PS Vita since it will be on there, but I rather get a 3DS for the games it actually has and will get. As far as I know it won't have a play to win system with it's micro-transactions since all I've seen has been for skins and accessories.
Official Site: www.pso2.com
Check out http://www.pso-world.com/ for all rumors, news, etc. It's the biggest PSO fansite!