Where in Europe is best for travelling behind diesel loco hauled stock?
Depends what you want as a priority really, there are loads of choices.
Just loud diesels?
Long distance IC services?
Guaranteed (or high chance) of fresh air stock rather than aircons?
Decent place(s) to stay/go/be?
Amount and variety of trains/locos/routes?
The most rateable diesels
I'd say you can have these days are Czech 749s (and they are going) or former Yugoslavian GMs, but I've barely scratched the surface of what is available.
Youtube is pretty good for most places, both British and local cranks tend to have a bit of stuff on there, also railfaneurope.net has a good basic guide to fleets.