I'm not going to try to come up with suggestions for each Pokemon, considering at this point no one is experienced when it comes to the fourth gen (of course except for the lucky few who have played the Japanese versions)
One thing I know about Leafeon is that it has poor Sp Defense, so use sunny day with caution. Dig would make a nice surprise for Fire types, just be sure they aren't packing earthquake or you could be in for a surprise yourself
Dusknoir I heard is about the same as Dusclops in Gen3. It makes up for its loss of Shadow Ball with improved stats. A 100 attack makes it a decent attacker plus pressure is still there to drain PP, and slightly improved defenses make 'noir a pretty good PP sponge. Bannette unfortunately got the shaft this gen. I'd go with Dusknoir.