The best?
I've seen Saxon three times, was going to be a 4th supporting Motorhead but Lemmy's still not 100% and healthy enough to tour so the show was called off T_T they're my favourite live band
Other top bands I was blown away by include Chthonic, Heaven & Hell, Alestorm, Dragonforce (people told me they were poor live, they were class!!) Wintersun Airbourne and Turisas

Now I've also seen some terrible bands, opening for Heaven & Hell was Iced Earth who at the time were mediocre because of their singer (Tim Ripper Owens, he just never fits into any band he sings for) and Lamb of God who have to be right up there for worst of the worst, nothing but a droning wall of noise.
Trivium are also pretty bad live but when I saw them they just released the Crusade and apparently they've gotten better since...
The Defiled are pretty bad live, none of the band can stay in time or key.
I also saw a really really bad Glam wannabe band called the Treatment as well.

So I've seen my fair share of good and bad live.