For me, in no particular order and giving as best a geographical spread
J15 aka 101 - the original all purpose, standardised, go anywhere locos.
NCC 'Jeep'
GM121 the pioneer GMs
MGWR Class G2 - last 2-4-0s here or across the water and flying machines to boot
GSR 500 class
GNRI Class V
Honourable mentions:
GM 141/181
Woolwich Mogul
(Rebuilt) A Class Metrovicks
Overrated:
800 class
Worse than useless:
Sulzer 113 class