Hmm, these would be my favourite fights:

Genins:
Naruto vs Sasuke (just something what would be really intresting)

Chuunins:

Iruka vs anyone (I'd just like to know how good Iruka really is when he gets serious)

Jounins and above:

Jiraiya vs Orochimaru (This would be a real badass fight. And we haven't yet seen Jiraiya in action, and Orochimaru also hasn't shown his true powers yet i think)

or

Kakashi vs Kabuto / Gai (Well, we haven't seen Gai using any special jutsus yet, and I think it would be intresting to see some. Also I think and hope that Kakashi still has some super unigue and powerful justu he hasn't yet shown)