OHL > WHL > QMJHL, maybe even >> QMJHL.
Stats in the Q mean the least to me because the defensemen and goalies in that league are really just... Let's just say they leave a lot to be desired. Seriously, name me 10 d-men that have made it in the NHL over the last 20 years. There's not a lot and of the ones who made it, there's not a lot of big names. And then there haven't been a lot of goalies making it either. Just look at the highlights from a few pages ago of Daniel Audette's six point night. It's not uncommon to find a few teams just like that when it comes to playing defense.
The OHL to me is the most credible league for stats because there are always really, really good goaltenders and d-men playing there every year and there are more players drafted out of the OHL than anywhere else. If you're playing in the OHL and you're able to score twice on a goalie like John Gibson (back when he was in the OHL), that holds a looooot of water for me. The OHL doesn't have weak years come the draft, they always have strong years. The WHL rarely has weak years, but they don't churn out the cheer amount of players the OHL does. But to the WHL's credit, they develop more d-men (I believe so anyway, this is purely from memory) than anywhere else.
By memory, I believe the OHL has the highest goals / game of the three leagues, followed by the Q and then by the WHL. But stats, while their relevance varying from league to league, always need to be taken into context. An overager putting up gaudy points always means **** all to me because there are always overagers that run the leagues because of their physical maturity and age. It's one of the reasons Connor McDavid, before his injury, putting up the stats he did was heart stopping. He was on pace to have the best season (point wise) in OHL history... At the age of 17. Unreal.