I think it's close but I'd go with Russell. The group that went after him is just about as good as the ones after Mandarich. Russell went 1st overall and barely played in the league. At least Mandarich played a handful of seasons.
There's several angles to look at it. When thinking about it more, I actually see the biggest differences between them as things that didn't stand out as badly for one bust as they did for the other 2.
Mandarich: A bust at OT doesn't set you back the same way it does at QB. The positional element made Leaf and Russell worse busts than him.
Leaf: Basically my previous post where the players passed over to draft him were nowhere near the level of ones passed over to draft Mandarich and Russell.
Russell: He had lower expectations than the other 2, who were expected to be franchise players. Sports Illustrated deemed Mandarich the best OL prospect of all time when he was on their cover, and Leaf was debated as the #1 pick against
Peyton Manning. Russell was in a weak QB class, and the Raiders were desperate for a QB. Anecdotal, but I remember a fair share of people (myself included) who were baffled that he was picked #1 at the time.
If making the biggest case for each of them being the #1 bust, here's what I'd say. Apologies for being redundant on some things.
Mandarich: All about who went after him. Arguably 2 GOATS at their position and an elite pass rusher in Thomas. Plus him being hyped up as the greatest OL prospect ever by SI.
Leaf: Played the most important position (unlike Mandarich) and had a lot more expectations than Russell. Made even worse by the Chargers trading up to draft him.
Russell: 3 players who were the best at their position in their time were drafted in the next 6 picks. Also his rookie deal was the main reason rookie salary limits were instituted in the next CBA.
I'd probably go Leaf at #1 due to his combo of position value and hype. But there's no wrong way to rank them, as different people find different things more egregious.