Cell and Frieza win effortlessly. Goku explicitly stated that Base Rild was stronger than Buu. Although the specific Buu he was referring to is debatable, I'm guessing it was Buuhan; it wouldn't make much sense to say "character a is only stronger than character b under certain circumstances" when such remarks are presumably meant to show how powerful a particular character is.
Frieza and Cell are likely several tiers stronger than Hyper Rild, as their combined efforts fared decently against a Goku who had powered up substantially since the events on M2. An array of blasts from Frieza even caused Goku to wince. The former then held his own quite well during a brief bout of hand to hand combat. Just these two factors alone strongly support the speculation that Frieza / Cell > All forms of Rild > All forms of Buu.
Bonus Round: Goku one-shots... Both Rild and Buu are fodders at this point; adding their strength on top of Cell and Frieza's probably wouldn't even help their chances against Base Goku.