Can't say Dave Mustaine's vocals are the best during this performance, but he hasn't ever really been known for the strongest of live vocals. The guitars rock. However, this performance is why Metallica will have an advantage over Megadeth in the mainstream.
Metallica has that groove they find and they are much more energetic live, whereas Megadeth's shit is so technical that they are more focused on playing accurate than entertaining.
Just my two cents though.