By me, Atlantis, followed by JO. Or, to amend that statement, anything that I've just bought or restored, but over time, it tends to go back to those two the most.
By my kids/future son in law, they tend to gravitate more to Grand Prix, so much more that I moved it to my daughter's house so they could enjoy it more.
When I owned Pinball Pool, the younger generation seemed to like the electronic game over the EMs, but when it's all EMs, GP seems to be the most popular.