Favorites:
Thirsty for the increasing difficulty and attempt at a PvP-type mission.
Thunderhead for the monsters that are considerate enough to come to me so I don't have to chase them around.
Hell's Precipice from the portals to the end.
Dragon's Lair is always a blast, especially trying to beat down Glint.
Most Hated:
Elona. Why is there always one party member who thinks they can run it, die while bringing about 10 monsters with them, then blame the party and leave?
Ring of Fire. Similar to Elona one person always aggroes monsters, brings them under the ether seal the party is destroying, and wipes out 7/8. And that person is the one living, and of course did not bring a rez sig.
Thunderhead when I join a PUG of rushers. I guess they're bottlenecked there for a reason.
[ Off topic ]
One thing I have noticed about Thunderhead though: Always start the party yourself. Before enterring the mission, discuss tactics with the team. Anyone who spams "gogogogogogo" is a rusher -- kick 'em fast. Anyone who never types anything is a possible freeloader or they don't read team's messages. Make sure they actually understand English (hey, never know) or aren't simply afraid of being flamed for saying something "noobish". If they are the previous boot 'em. If latter, they're probably a good player in disguise. Last warning sign is the guy who insists his Axe/Smite/Heal/Tactics/Strength tank is teh uber pwn and won't decide on three (or four...I've seen it work) attributes is not a team player. They'll probably be the ones to watch the team die, spam some obscenities, and map. Again, kickity-kick-boot 'em beforehand. A lot of folks in Thunderhead have been trying to pass that mission for a while. Anyone willing to spend an extra 5-10 minutes discussing tactics is probably a good choice.
[ /end off topic ]
On a side note: Most abused mission is Sanctum Cay. I usually get good drops there (silver and black dyes, golds, scrolls) so I sometimes hang out waiting for a good looking party to request one more.