You have the sanctioning bodies (WBA, WBC, IBF and WBO). But they won't rank champions of other sanctioning bodies. So they probably aren't reliable.
Then you have The Ring rankings, Transnational Boxing rankings and Boxrec rankings.
But out of those 3 I am not sure which is the most respected.
TBRB
boxrec is just based on points and the formula that they use to make their rankings results in funny rankings. Just checking their top 10 HW ranking right now:
1) Usyk
2) Joshua
3) Fury
4) Wilder
5) Dubois
6) Wallin
7) Zhang
8) Joyce
9) Whyte
10) Gassiev