Wins and losses are the usual and obvious answer, but sometimes others have to be rated differently in my opinion.
I will use Vancouver and Gillis for an example.
Gillis in his 4 seasons as GM has gotten 100+ points every season with 2 (back-to-back) president trophies. Do we judge him as a success because of this? Maybe not because he inherited an extremely good core. For him (and he is one of a select few who can be judged in this manner), he may be judged on whether he brings a cup to Vancouver or not in his tenure as general manager instead of just wins and losses.
Gillis got to inherit a stacked roster in Vancouver (built by Burke and Nonis). He has added no significant pieces really, just depth guys for the most part. Apparently he is a good GM though.
Hell, if you made me the GM of the Pittsburgh Penguins tomorrow, I would get my team into the playoffs every year, and maybe get a cup or two. Does that make me a top end GM? Nope, I am just riding the coattails of someone else.
Burke inherited a JFJ built roster, which was filled with old guys, bad contracts, and guys who quite frankly weren't even good. In that time he has revamped our entire prospect pool, and built a significantly better roster. I fault Burke for hanging on to Wilson too long and a couple other things, but you cannot question that this franchise has a vastly better skill base than before he got here.