It's a little bit biased, but I'd go with the Friedman/Mustaine combo.
I was fortunate to see 'deth on the Youthanasia tour. The Friedman/Mustaine/Ellefson/Menza lineup still had to be my favorite one of all especially since Rust In Peace is now in the classic thrash metal canon.
It was also my first ever major concert when I moved to the US. It was also good that Corrosion of Conformity opened for them. Swak na swak yung band lineup para sa akin. I also can't believe that was more than 10 years ago for me.