I don't think anyone else mentioned it, which surprises me, so I will;
Old Axe Armour Mode.
Seriously, that last mode was so hyped (before we knew what it was). What could it be? A John and Eric coop mode, Simon (considering it was the 20th anniversery), or perhaps a Wind mode? Nope, you get a totally character-less, meaningless and extremly boring mode. The other modes weren't really much better (excluding the REAL mode).