Hmm, Stars this is kind of an open question because I think Max and Liz saved each other many times.
I think Max saved Liz at the Crashdown, and he saved her when he tried to heal her when she had the sparks(oh, that was in vain, maybe we shouldn't count that one

), he saved her when "he" came back in The End of the World...he saved her and himself from heartache.
I think perhaps one of the most controversial ones for me was when he saved her in Departure...it wasn't an immediate effect and he needed a lot of waking up before he did it right and proper (by of course getting rid of Tess). I think Liz saved Max in Blood Brother, and she also saved him the countless times she protected his secret. I think she saved him when she rescued him in Destiny.
But if I had to go with one of the most direct ways, I'd pick Max saving Liz at the Crashdown.
OKay, how about this one:
Best spot for a Canon fic to pick up with our Dreamers:
End of the World or Destiny?
"The expected is just the beginning. The unexpected is what changes our lives."
Meredith - Grey's Anatomy