At the moment, I'd say the Bit.trip games, but I'll be playing the Trauma Center games soon, so that could change. Mega Man 10 on Hard mode also gave me quite a bit of trouble for some reason, even though the Wily fortress wasn't all that special except for some frustrating gimmick parts.
But IMO there aren't really that many tough games this gen, at least when compared to last gen games, like F-Zero GX.
Warning: The preceding message may or may not have included sarcasm, cynicism, irony, full stops, commas, slashes, words, letters, sentences, lines, quotes, flaeed gramar, cryptic metaphors or other means of annoying communication. Viewer discretion is/was strongly advised.










