I felt that Grand Fantasia was a really nice and solid MMO for a F2P game. It had good controls, good events, and a fairly nice community compared to a lot of other games, though I haven't played the other two games so I don't anything about them.
Grand Fantasia - piece of asian sh1t.....
Cardmon - played til lvl 4, didn't like that it's based on the card system. U're nothing but your cards.
Iris - played a lot, had 37 ranger and was really enjoying it until there were no quests to do (it's sick to lvl without them)
SUN - played through OBT, was quiete nice, pvp zones are hardcore due to equipment drop. I've just D/Led it again and will probably give it another try.
I haven't played the other three, so I will just talk about CardMon Hero.
CardMon Hero is a pretty decent game, and very unique because of its card summoning system. I really like summoning stuff because I like the feeling of battling with something else besides my main character.
This is the mmorpg that I am currently playing. You can play this too as a side mmo to other more serious MMORPGs if ever you're playing one.