68's look, sound and perform great. I loved the sound of old Class 50's and 37's, but a 47 has a distinctive sound too. Never saw Deltics in the flesh. And HST power car in any guise, Valetta, VP185 and MTU all look and sound great. I know some lament passing of turbo whistle on Valentas, but the MTU has a distinctive growl, and is so much cleaner. I do not miss the amount of smoke that Valentas produced starting up or on pull away.
Love the look of Class 90's, but they seem a bit light footed and struggle in the wet, but on a dry rail pull away from any other electric or diesel loco with ease. Class 91's have a very futuristic look and 140mph but somehow seem to lack some soul. For me the 88 is the best electric loco, looks great, and has the last mile bi-mode capability. Class 68 a close second.
But given a chance to choose any loco to pull a train (if UK loading gauge were to continental standards) would have to be the (broken nose) French Class BB22200 - 125mph capability - dual mode electric 25kv and 1.5Kv DC (you could travel behind a single loco from Calais to Nice starting on 25kv AC down to Paris, switching over to 1.5KV DC on the PLM, back to 25KV AC from Marseille along the Cote D'Azur. , the monometer bogies have a distinctive motor whine, and they pull like no other loco i have ever travelled behind. Those locos could whisk a 14 coach train to 100mph very quickly. The French BB26000's come a close second.