Endless Mode would probably be the hardest because it takes so long. It isn't difficult like get-to-level-100-on-Robotron difficult, just incredibly time-consuming.
Puzzle Mode was probably the easiest for me, either that or Power10000. (It's much easier to create power gems without really trying than to create hyper gems.)
8 Retail 1000s, 1 Retail 250, 11 Arcade 200s, 1 Arcade 250
360 in a day: 22 Jan 2008
Best achievement: Virtuoso on Rock Band (finished vocal solo tour on Expert, 50 GS)
waiting for Skyrim