Ugh... Gamer, by a longshot. Getting randomly hit by the Mom in midair while you thought you were in the "safezone" is no fun at all, and getting barely knocked out of the safezone is even worse. It's like Norfair, but even more stupid. Not to mention, the stage itself is borderline random not only in terms of layout, but when and where the Mom shows up. Worse of all, it only has, what, TWO good tracks on it?
Orbital Gate Assault and 75m both get dishonorable mentions as well, the latter because STAGE HAZARDS EVERYWHERE WHY DID THIS STAGE RETURN AAAAGH, and the former because it's often unclear what's actually an element of the stage and that giant freaking explosion at the end.