I'm very surprised.
I love Wesnoth and it's very original. It's also a full game without missing features. But then of course I hate the single player experience of it - I only play multi-player. Out of the three game there that I have played, I would rate it at the top.
Also, I can't judge FreeCiv on it's own, I have owned Civ 2-4 and I always compare FreeCiv to those games, and that just makes it feel so old and outdated.
Haven't played the other two.
I suppose at this stage UFO:AI is the opposite to Wesnoth for me: I play single player but I am yet to play multi-player (I am one of those lazy people that doesn't play multi-player unless there is a dedicated multi-player-server-tracker).