Corbyn delivered the fifth largest number of votes for a post-war Labour leader in a General Election,the second largest since 1966, and the largest for a Labour leader that won neither the popular vote nor a plurality of seats:
Attlee 1951 - 13.95m (L)*
Blair 1997 - 13.52m (W)
Attlee 1950 - 13.23m (W)
Wilson 1966 - 13.10m (W)
Corbyn 2017 - 12.88m (L)
Attlee 1955 - 12.41m (L)
Gaitskell 1959 - 12.22m (L)
Wilson 1970 - 12.21m (L)
Wilson 1964 - 12.21m (W)
Attlee 1945 - 11.97m (W)
Wilson 1974 - 11.65m (W)
Kinnock 1992 - 11.56m (L)
Callaghan 1979 - 11.53m (L)
Wilson 1974 - 11.46m (W)*
Blair 2001 - 10.72m (W)
Corbyn 2019 - 10.27m (L)
Kinnock 1987 - 10.03m (L)
Blair 2005 - 9.55m (W)
Miliband 2015 - 9.35m (L)
Brown 2010 - 8.61m (L)
Foot 1983 - 8.46m (L)
*Indicates that the party with the highest popular vote did not win the largest number of seats
Averaged out across their elections, the leaders rank as follows:
1. Attlee - 12.89m
2. Wilson - 12.55m
3. Gaitskell - 12.22m*
4. Callaghan - 11.53m*
5. Corbyn - 11.58m
6. Blair - 11.26m
7. Kinnock - 10.80m
8. Miliband - 9.35m*
9. Brown - 8.61m*
10. Foot - 8.46m*
*Indicates only a single election contested
Unless you were born in 1979 or earlier, Corbyn is actually the most popular Labour leader in your lifetime (I'm 1977, so aside from Callaghan's one-and-done, he is in mine).
I'd also argue he's the best, because he is actually Labour leader - he is left-of-centre and slightly liberal, which is contradictory but core Labour (it's no less contradictory than the majority of other parties; Labour mixes fiscal control with social freedom, Conservative mixes fiscal freedom with social control. Regional nationalist parties are usually control on both axes, and the Lib Dems wander around the centre like an asteroid captured in an eccentric orbit).
Miliband wasn't actually that far off, but Brown and Blair were very crony capitalists of the worst kind, in red ties. Kinnock's makes it across the centre too, and so did Foot - but fighting an election based on disarmament shortly after the UK had defended the Falklands was insane.