Have always loved going to Hampden despite the fact that getting there can sometimes be a mission.
In terms of easy access, I'd say Ibrox is probably best.
Access to Parkhead also used to be difficult. Has it gotten any easier in recent times?
Well you can't access one of the two Ibrox junctions from the M8 like you used to, but in return there is now a motorway to the south of Celtic Park as well as the north, as well as a new road that runs you to the car park. That new road extends south and pretty much takes you to Hampden too. So I'd say in terms of driving it's now easier to get to Celtic Park than it is to get to the other two, but really they're all pretty good now. You just can't park at Hampden!
Obviously Ibrox has a the Underground which the other two don't, but there's four train stations all pretty close to Celtic Park (Belgrove, Dalmarnock, Duke St, Bridgeton - though give those last two a miss if you're a home supporter), while Mount Florida, Kings Park and Cathcart are all handy for Hampden. Not sure there's much in the way of regular train stations for Ibrox.