I enjoy Glory of the Gods (multiplayer), and I also had a lot of fun with world3 from
Illwinter's map page. I tend not to use random maps, or if I do I generate one and use it multiple times. Of course you're aware that you can still use Dominions 2 maps?
Edit: one more thought. You're aware that when the AI has a really big army, it tends to rout before they're all dead. If you cut off his retreat provinces the same turn, all those routed troops have nowhere to flee to and die. This can be the difference between, say, 83 out of 284 enemy troops killed (and 0 of 6 commanders), and 284 out of 284 killed (and 6 of 6 commanders, some of them mages). This is another reason it's important to be able to split your army when necessary, so you don't have to keep defeating the same army over and over again (until it gets whittled down small enough to be defeated by PD). BTW, if you cut off the retreat provinces the battle will still tell you that 83 out of 284 were killed, but you'll know by looking at the map that the other 201 died, too.
-Max