Warioland on crack. The number of times I was truly sweating not knowing if I'd zoom to the finishline just in time were too many to count. It rides that fine line between just difficult enough to keep you on your toes but I never lost a level completely (aside from WAR) so can't say it's overly hard.
The style alone is impressive to say the least, the level design is ALWAYS fresh utilizing a new game mechanic to constantly mix it up.
Add onto this the eclectic cast of characters and just WTF style humor and it really is a modern classic in the making.
Can't wait to see the speedruns that this game was built for.