I think that any of the following scenarios is possible
1) Roger = WB>Garp
Hype:-WB was the only equal to Roger
2) Roger >WB=Garp
Hype:- Garp and Roger almost killing each other several times, so it may be so that the only pirate that could have matched Roger was WB and the only Marine that could match Roger was Garp. They might be very near to Roger's level, but still a little weaker.
3) Roger > WB > Garp
The above two statements maybe pure hype and it maybe possible that Garp was never that great to begin with.
I don't know what is the truth, but I think that whatever it is Roger has to be on the top. (No shit I know, but can't conclusively decide based on hype)