Benno,
depends on many things, first, should it be a big battle or a small one. I learn for myself that the small ones are usually the more interesting ones for a diorama. Another question is perhaps "who should be the winner"
, but far more important what units do you want to display, and how realistic will you do it. For example there are many "battles" fought in the woods, so you will see not much of your little friends if you do it right
.
For me cowpens will surely be a very good choice if not number 1!
http://www.britishbattles.com/battle-cowpens.htm
Also Guilford Courthouse (strongly wooded)
http://www.britishbattles.com/battle-guilford.htm
and Camden
http://www.britishbattles.com/battle-camden.htm
are nice ones.
If you need numbers for one of the above mentioned fights look for the free wargame rules, there are some that gives good BO´s. or ask, perhaps i have some BO´s, depending on your choice.
regards
Herbert