Crosby might have a claim for 5th best player but certainly not 5th greatest.
Greatest means accomplishments, personal or team, without considering other factors. That he was injured and missed out on some personal awards affects his greatness.
Hull, Beliveau, Richard, Harvey, Roy and Hasek are all good choices for 5th best.
The thing Crosby has going for him is that he has no weaknesses in his resume.
Peak? Great peak. No it's not as good as it could have been - but against most of the usual top 10-15 candidates it holds up nicely still (esp when you have Beliveau, Harvey or Roy in there)
Playoffs? Great playoff resume. He has the smythes, the cup runs, the overall consistency - again he does very well here against the usual top 10-15 players
Prime/Consistency? He's actually one of the best ever here - probably top 3 with Bourque/Howe, maybe Gretzky.
International resume? Yep, he's up there among the best ever.
Leadership/intangibles? Harder to quantify, but was youngest captain, led teams to victory at every level as captain, great overall reputation too.
Awards? He has plenty. Not the most it's true - but compare him to Beliveau for example and he does super well, and Beliveau is a very popular #5 guy
Finally - "greatest vs best". It's tricky to compare the 2. But i'd argue Crosby has a better case for 5th greatest, than 5th best. His peak (especially if you consider what could have been) is fantastic - but to be the 5th "best" would mean he has the 5th greatest peak. He doesn't. Guys like Hasek, Lafleur even, maybe even Ovechkin or Jagr, Hull - many players in history may have been "better" than Crosby at his best, or for longer. He's great because of his consistency mostly.
To be clear i'm not saying Crosby at #5 is indisputable - I just think it helps his case how well rounded his resume is, with no glaring weaknesses. Beliveau is very much this way too.