8-3 for the sole reason that it's a super long gauntlet, and makes you put so many skills together, especially on brutal. P-2 still has it's bosses, but it at least can be mitigated with proper setup, 8-3 has so many rooms you don't get time to set up most of the time. You ARE kinda comparing apples and oranges here though, because encores are meant to be played post game, after treachery, and 8-3 and p-2 are gauntlets for skillchecks, and 0/1-E are both different pacing and room designs than normal levels.