The Mercurians, because of the several inherent annoyances: If you want to play as them, you first have to play as somebody else for about 300 turns. You can't set them up in your capital- you need a second city capable of producing a wonder, so you have to at least nominally play for all those turns. Whether you switch or not, after building the gate you are shackled to the AI corpse. You find yourself wanting to sabotage your empire by deleting all units and declaring war on the Hippus on the turn before gate completion. It's an unwholesome desire.
One idea which I endorse would be the inclusion of a Mercurian placeholder civ, a bunch of angel-worshippers geared to summoning Basium. The angels produced by the gate could be under their direct control. Who controls Bourne The Gleaming at the start of the Radiant Guard scenario? I want to play as them.
Also, I wish Basium were charismatic instead of raider, since the angel mechanic places more emphasis than usual on individual units, and angelic good seems like it should be more "glacial, inexorable purification" than "burn some farms for quick cash and retreat."
The Sheaim also get a vote, because even though I like their whole ultimate evil thing and the planar gates, the satisfaction to be gotten playing as them is directly tied to how awesome Armageddon is, and Armageddon sucks.
The summoner trait is sort of awful. Two extra turns of life could be useful in some situations, but why not just have it make the summons permanent? This would make it the hot trait of the year, so I guess that's too much to ask, but it would really cement their image as a race of monsters.
Final vote is for the Luchiurp. I've never really played as them, but I'm tired of accusatory whining about all of their wonderful creations while I'm still trying to build my first settler.