Well there is not such thing as "best" battle for me ....but!
Strategy wise the best was Gokudera vs Belphegor, as I was unsure until the last minute what was going to happen and it also showed the strong Vongola bonds.
Power show-off wise the best one was Tsuna vs Xanxus as we saw power levels we hadn't seen up to that point.
The coolest one was Hibari's fight because it was Hibari's fight ofc :P
But the one I voted as my fave is the Lambo vs Levi one. For me this is the one that the real Vongola powers, their bonds that they place above any prize or power ranking, is really shown when Tsuna stops the fight to save Lambo.
With their dreams as their guide... they are sailing for the stars!