Hands down without a doubt, mares. I've had one gelding and one mare and the difference between the two is astronomical. It probably has more to do with the individual horses, but I know some of it can be attributed to gender.
My gelding was even tempered and kind, but he was not affectionate and I never felt that he looked forward to seeing me or being with me. He was extremely lazy, slow, hard mouthed and not sensitive at all to my legs. I had to use huge spurs and a whip and since I was ten at the time I was miserable and felt like I was abusing my horse. I'm a very soft rider, too soft (my trainer says I have jello hands) and I wanted a sesitive horse that I would match with.
Well eight years later, I got my wish. Whether its because she's a mare or not, I'm not sure, but my new horse is incredibly sensitve to my aids. She's very quick and constantly ready to go at the feather touch of my heel.
But above all I love that she comes when I call her, nickers when she sees me, and presses her face against my chest for a rub. She was raised with a lot of attention and she seems to prefer me over other horses.
If she's any kind of a representation of mares, then I am a mare person through and through!