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5x5x5, followed very closely by 7x7x7. Nothing else is even close. Probably next would be 6x6x6, then 4x4x4, then 2x2x2, then finally 3x3x3. I hate 3x3x3.
That's for speedsolving. BLD is different, but I'd still say my favorite size for BLD is 5x5x5.
7x7 comes second, not an even layered cube so it's a barrel of fun to solve and you feel good when it's done xD
4x4 is horrible, probably only slightly faster than my 5x5 times due to really terrible edge paring cases that I get. A good solve would be 1:30, a bad solve 2:30 due to getting edge parity early.
my 6x6 popped before I could solve it the first time, need to hunt for the piece. I imagine it would be between 7x7 and 4x4.