Oooh oooh I'll bite!
It's obviously the good old SR! Why? Because GWR, LMS and LNER have their own fans who think theirs is the best. SR gets left out in the cold and therefore it's the best. Oh and the fact some of their locos were built in Brighton and that has to count for something.
Of course you can't go wrong with the world record holder when it comes to speed but i think i'm right that the last British steam loco to break the 100 mph barrier was a Battle of Britain class loco in the mid 60's.
Oops, close but no cigar, it was a merchant navy which last broke the 100 mph...