For childhood nostalgia purposes (not necessarily thanks to their being powerful and/or distinctive in tone), Classes 156, 314, 318, 320 & 334. The latter three in particular, as those are the units I've travelled the most miles on as a collective group.
Putting childhood nostalgia firmly to one side, it has to be the 90s/91s/DVTs as already mentioned, just for being equally powerful and distinctive in tone. Another example of this would be the horn the 59s, most earlier-build 66s (the later-built examples have a different horn fitted, at least from videos I've seen) and the 67s have. HSTs are another example, just for how good it sounds.
Deltic horns always sounded like Southern EMUs, were they the same horn?
If not the exact same horn type, then a similar sound effect produced by different models.
What about worst, got to be a 47?
As distinctive as it is, I have to concede that the 47's horn isn't my favourite aspect of them.
Best sounding/Loudest i'd say would have to be the HST and Class 56.
I concur. The 56's horn is well-suited for what it is: a heavyweight freight loco.
180s too (turn your volume down if wearing headphones for this one):
and then there's the 158 which as we all know just sort of wheezes!
The 158/159 horn sounds like a wheezing parp more than anything else. Can certainly tell when one is approaching though if the horn is used!
You have to go to North America, where the classic is the Nathan Five-Chime, for a strident but eerie tone. Not all railroads install it, but it's often seen as the sign of one which is not impoverished. They still call it a whistle.
With the equally classic Long-Long-Short-Long blast for a level crossing (or progressively, for a series of them from the far distance as it approaches), it's been described as "America's alternative national anthem".
On the subject of North American horns, the Leslie A-200s fitted to the legendary Pennsylvania Railroad GG-1 electrics are right up there. They sound more like ocean liner horns than locomotive horns:
