Personally, I have always liked Death Island for race because it has such a circular track-like shape. Blood Gulch does not seem as well-suited for rally to me because you are constantly being blasted out of your ride due to the close quarters and small size.
Infinity is pretty fun too though because of the incredible expanse and range possiblities for warthog weapons.
For Capture the flag, however, Blood Gulch has always been my favorite. It seems incredibly well balanced and I love the placement of everything such as the weapons/items, the cave, and the cliffs.