McGwire.
Firt of all, let’s get the timeline straight; Canseco didn’t show up, McGwire did. Canseco arrived before McGwire. (And on other nitpicky notes, Foster had one 50-homer year, 1977. Stargell and Aaron had none.)
Secondly, it’s a fool’s game to try to divine steroid use by statistics. McGwire hit 49 homers as a rookie. That’s 11 more than any other rookie season ever; clearly, the guy was not like other guys. A lot of steroid users are scrubs with no big home run years at all. If you ascribe any home run burst to steroids you’d have to conclude that Babe Ruth was on steroids, and Mickey Mantle, and that steroids suddenly became less popular in the late 60s. Hank aaron, who suddenly had his best home run percentages in his mid-to-late 30s, apparently discovered steroids late. You’d also have to conclude they were very popular in 1987 and then everyone stopped taking them in 1988, when home run totals dropped about 40%. Or you’d have to conclude all the pitchers started using 'roids in the 60s, but stopped using them in 1969.
Statistical standards in baseball are not consistent. In 1908 the average NL player hit .239 with one home run and made twice as many errors as a player in 2008. Home runs went from being rare events to commonplace between 1919 and 1929. Stolen bases were discarded as a strategy for thirty or forty YEARS before suddenly coming back into vogue in the 60s. The AL batting avedrage in 1968 was .230; five years later it was .258.
You can’t just assume a guy who hits a lot of home runs is on steroids (and you can’t assume a scrub who doesn’t hit a lot of homers isn’t.) Home runs are up today for a lot of reasons; ballpark construction, changes in the way bats are made, weight training (independent of steroids) and hitting approaches and philosophy.
Similarly, I’m leery of using a guy’s physical appearance as proof he’s on roids. (Barry Bonds, okay, is an extreme case.) Rafael Palmiero didn’t look like he was on roids but apparently he was. A-Rod (henceforth “Oompa Loompa” for his scary orange skin) all lean and tall, doesn’t look like a roider, but he is. On the other hand, Frank Thomas was built like an Abrams tank but I’ve never heard so much as a whisper he ever took steroids. Guys do get bigger and more muscular into their thirties - “old man strength,” they call it in strength sports like shot put and hammer throw. And you can take roids and apoparently not look like a cartoon, as Palmiero and Oompa Loompa demonstrate.