Over the years I have had plenty of cab rides, official and unofficial, steam, diesel and electric.
Can't say I am keen on steam far, too dirty; spent a day acting as pilotman (for a signalling disconnection) on a preserved railway once - it took two showers to get clean.
Worst riding thing I have ever been is a toss-up between the old class 81 and 85 electrics, awfully rough and bounced all over the place and an Irish Rail 141 class - shook and rattled like hell
. 86/0s could be interesting as well though the other varieties rode better.
With regard to the opposite BR 80xxx 2-6-4T would probably count as the best steam loco and the
Swiss BLS Ae6/8 electric was quite smooth for it's age - probably all those wheels!