This is actually quite a challenging question, when I read the thread title my immediate answer was the IETs.
However, I then read the thread and its replies, which reminded me (I've only just had a coffee while browsing the forum, not yet fully awake after NYE!
) of 68s and 380s. I do remember commenting at thw time before I travelled to Scotland about not pointing my camera at a 380, to make sure it didn't break because of the look of a 380 front end
Of course, that was before I'd even seen one. 2011 was it when they had entered squadron service? I remember being pretty impressed by the 380s when I rode on one to Gourock, and I still think they're pretty good. Not been on one for years though, I really do need to get back to Glasgow!
68s of course, well they're just way too much fun! That noise, especially through the tunnels in Edinburgh, the closest I suspect we'll ever get to a modern version of a 37.
700s, yeah they're good but not my favourite. Can't comment on the likes of 374s, 755s, 385s or 230s as I've not sampled them.
So yeah I don't know. For pure unadultered fun, the 68s are a clear winner, but I'd also have to vote up the GWR IETs.