For me, it's got to be Beta Alanine. I had heard about it, but didn't try it until March.. With it, muscle endurance is noticably up. Just always able to push those few extra reps when it's needed. I also have much less severe DOMS.
A close second would be Cissus over Glucosamine/MSM.
Mt Snow, VT - May 5, 2012 - Completed!
Gunstock, NH - June 1, 2013 - Completed!
...i remain, he who remains to be...