Thanks for the clarification, it does affect my vote.
If it were "best", then I would vote
(1), express passenger A4, fast and powerful
(2) mixed traffic the V4 (better route availability and riding, and more even torque than the B1)
(3) freight the P1, the 9F of the twenties and thirties but a loco ahead of its time.
As we are voting for most successful, my votes are
(1) express passenger A3, (more numerous and longer-lived than other express classes)
(2) mixed traffic, V2 (numerous and versatile)
(3) heavy goods, the Robinson O4, a very numerous, long lived and robust locomotive.
Whilst I've not included any tank engines, in my mind none challenged the above choices.