The answer depends on who you ask - enthusiasts or non-enthusiasts.
Non-enthusiasts will have barely noticed the difference between the different diesel classes, especially when they were all painted in allover rail blue. Some will probably recognise HSTs, but you have to remember that much of the country never had any regular HST services, and the only time they would be seen was very rare appearances on TV.
Enthusiasts mostly have their own individual preferences; some like Deltics, some like Westerns, etc., etc. Personally I have no favourites, and even a bit of a love-hate feeling because diesels replaced steam.
As for Flying Scotsman -- maybe the reason it is so popular is that - apart from looking quite impressive - it is the only UK steam loco that has been allowed to operate on the main lines almost continuously since the 1960s **
(** Apart from its disastrous overseas tour, and longish periods during overhaul.)
I suspect that Mallard might have been equally famous if it had been allowed to remain operating, but apart from a brief spell circa 1975, it is apparently "too delicate" for regular use, and NRM did not wish to pay £££,£££ to get it working when other A4s were available.