We will be renting a car for this trip and are wondering which one of these hotels has the easiest parking garages access/highway access - or which one to avoid - Harrah's, Bally's or Flamingo.
Pretty much what CLV said, I don't think that any one of those is easier than any other.
I don't know where you are staying but if I am staying elsewhere and looking for a place to park in the mid-strip area, I usually park at Bill's. I have found that I can be in either Flamingo or Ballys casino faster parking at Bill's than if I used those casino's parking lots.