Muskie I think could have won '72. Humphrey's already been discussed, Wallace was... Wallace, Jackson was too militaristic, and Chisholm would have been fallen to the same trap as McGovern, being easy for Nixon to paint her as a "pinko peacenik".
Muskie could have won '72, you say. Probably not. His anti-war stance would have gotten him painted as a liberal extremist just like McGovern. Moreover, there's no way Muskie would have had enough (any) appeal in the South or border states to garner some of those electoral votes and that would be the end of his election chances.
As I noted in Post #4, Humphrey is another "No." American voters normally don't support retreads, Tricky Dicky, though, being a notable exception. But ultimately, there wasn't any big policy failure during Nixon's first term that would caused enough voters to think, "Gee, I should have voted for Humphrey, he would've done that better!" Humphrey was so loved by his own Democratic Party that he got 25% of their votes in the primaries.
You next observed that "Wallace was... Wallace." Agreed. He'd do about as well as McGovern but drawing his support from a different constituency. That said, he'd probably get more electoral votes than McGovern by grabbing several Southern states.
You contend that Jackson was too militaristic. I think that was his strength in this race. He may not beat Nixon but he's the one Democratic candidate that could have been competitive. His foreign & defense policies cut heavily into Nixon's turf.
Lastly we have Shirley Chisholm. "Pinko peacenik" sums her up pretty well. Nixon's landslide would've been even bigger-- 534 electoral votes and ~68% of the popular vote.