oh man, camel archers, and phalanxes win big time. i'm not even going to consider explorers or ironclads, which i take to be useless for granted.
however, when you consider the most useless UU's, you have to start with the F-15. thankfully, that no longer exists. therefore, you now have to move on to the two worthless UU's... the phalanx and the camel archer.
the phalanx gets an extra combat point, which is nice. however, axeman still have a bonus against melee units, making the phalanx nothing more than a specialist with which to pack your armies against horse archers and whatever else mounted comes your way. so, essentially, they're no better than spearman except for the extra point, which hardly matters because any spearman can beat any mounted unit before knights hands down, in my experience. all in all, its not a useful bonus.
camel archers, on the other hand, have even less of a discernible bonus; they have a 25% chance of fleeing combat, whoopee. beyond that, they only have going for them that they dont require horses. that would be nice if it were a replacement for horse archers (or even chariots), but for knights horses are generally already a valuable commodity: otherwise, you're liable to be dead.