i'm gonna go and say the viper. it's painful with the loaned pewpew guns, but if you rip those off and put 4 nice guns (even all class 1's) on and grab some of the cheaper upgrades she starts to pull the cash in nice and fast.
i think i started close to 200K trade in for the viper too, so it was very close to stock. it's now almost worth a million.
but save the eagle, it's a great ship in it's own right and they fly differently. if i want to throw myself into a dogfight then i'll use the eagle, it's the most nimble ship in the game