Requirements:
Single Player: 1 event (End of level).
Battle: 1 event (Start position).
Race: 3 events (Start position, warp, warp target)
Treasure Hunt: 3 events (Start position, gem, end)
CTF: 3 events (Start position, base1, base2)
Others: Varies... most special gametypes take lots and lots of events, of course (omg street fight).

As for what actually seems easiest to make... it varies. Sometimes SP, sometimes Battle, sometimes Race. CTF I suck at and I've never made a Treasure Hunt.