Didn't the Diamonbacks get to him too in a WS game? Would you pick Rivera or MadBum to close out game 7 of a WS?
Didn't have to go outside of that series if you wanted to make that point (insert Randy Johnson for MadBum). Kim (D-Backs closer) had already blown/lost 2 games for them, including Schilling's first start - and Rivera was dominating on the other side, with a save and a win in 5 scoreless IP over 3 appearances till that point. D-Backs called on Johnson to get the last 4 outs, and he got it done. Yanks did the same with Rivera, and he did not.
Which is why it is hard to label best ever. As good as he was, MadBum never blew a series!
Having said all that, you're completely insane if you want to use Madison Bumgarner as the counter for calling Rivera the greatest (I assume the original intent was closer) ever.