Obviously WoW is going to get the most votes, they have like 90% of the entire MMO market. People will vote for what they know.

City of Heroes is the best though for PUGs and i'll tell you why.

It has a very in depth search system.

You can search all players on the server and narrow by archtype (class),
level range,
origin (though that's sort of pointless as it doesnt effect powers),
zone,
& meta tags (like people can have a "looking for anything" tag or a "looking for missions" tag or a "looking for TF" (taskforce) tag, etc..
you can also sort by all aspects

people also have comments they can put that will show up next to their info in the search.

The tell/messaging system is integrated into the search so its easy to ask if some1 would like to join your group.

all of which makes finding people for a PUG. and making a balanced team. fast & efficient.