Given that the 0-6-0 was the backbone of freight and other operations on British railways for the best part of 100 years, this is a large topic to go at! The Caledonian, the NER, NBR, Midland, to name but a few, all operated some excellent looking and very useful 0-6-0 classes. The Dean goods must surely be up there and I also really like the rarely modeled Armstrong or standard goods, although they were mostly gone by the 1930s. Early GC types would also be on my list including 23s, and what became LNER J12.
Cheers,
Alastair M