I don't know why but class 20s used to do it for me. I liked 52s 55s and 37s in that order but 20s were definitely my favorite. I have driven a 37 over NNR and I have been in more 66s and 60s than I've had hot dinners but for some reason the 20s hold a spell over me.

Only one candidate, nothing else compares for its time, the D11-137 (later class 45) No matter what train you worked it was up to the job ! And for very heavy freight trains class 60.

Class 33 for me.

One more for the "Westerns" from me, they looked and sounded the business, loved the liveries like Desert Sand, Golden Ochre, etc when they were introduced, and their names were so romantic!

My favourite Diesel loco's are the Deltics, Class 40's. But I like the type 5 loco's the the 56's and 58's as they where the moden loco's. I'm in to.

Warships - they're so ugly you just have to love them. ;)

Favorite loco's
 

My personal faves were the deltics as even before you got to the station/ depot you could tell by the drone if there was a delly about, absolutely unmistakeable. But lest we forget the pairs of class 20's (choppers) that used to run the Derby to Skegness summer trains... used to take them from derby to notts and back just for the haulage, very memorable too... anyone else agree?

its got to be the class 26 for me then the class 76 woodhead electrics i bet not many railfans rate them.

For me it has to be the Class 52 Westerns.
After spending many happy railway summer holidays in Paignton with some of my fellow apprentices during my teen years through the early 70's the sound of a 52 still brings back fond memories.
Travelling on the Peak hauled Devonian out of Derby and then switching to a Western at Temple Meads was the zenith of a young chap's summer holiday.:)

i don't know what any of them are called so i'll just say anything that clags.