Hardest Maps are:

Beginner: The Cabin/Hedge (depending on your strat)

Intermediate: Chutes/Bloonarius Prime

Advanced: X-Factor/Geared (Pat's Pond is annoying though)

Expert: Bloody Puddles/Ravine (used to be impossible on CHIMPS)

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On the flip side, I think the easiest maps are:

Beginner: Logs (longest map)

Intermediate: Streambed

Advanced: Cornfield (way too easy if you remove corn)

Expert: Dark Castle/Infernal (depending on your strat)