My two cents:
Gorsebrook - If buy-in by the universities for this project is felt important, then Gorsebrook would likely be the best site
but it appears that Saint Mary's wants their own stadium, and it is virtually guaranteed that the denizens of the deep south end would be apoplectic with latent NIMBYism if such a travesty were ever visited upon them. Gorsebrook would also be inconvenient to many people outside the southern part of the peninsula. For these reasons, I think Gorsebrook is a non starter.
VGH site - An interesting possibility. The VGH site is close to SGR and convenient to Dalhousie. Transit connections are good. In many ways, this is an ideal solution. The biggest objection here would be that the site would be unavailable until the Infirmary is expanded, and the VGH demolished. This would delay the construction of a stadium by at least 5-7 years. There would be no CFL team for at least 10 years.
Halifax Forum site - This also applies to other sites mentioned in the northern portions of the peninsula. Another very good choice. The land is flat and site preparation costs would be low. Transit connections would be easy to establish. There is the potential for on site parking for fans coming in from out of town. The site would also be fairly easily accessible to people driving in by car. This might actually be the ideal compromise location for a stadium.
Cogswell Interchange - I am concerned that this location may be too cramped and I am also concerned that this location (in the heart of the core) might be too congested for traffic movement as well. It is important to remember that a stadium is not the same as an arena like the Metro Centre. While the Metro Centre sees activity on many days of the year, there would only be about 9 home games per year for a CFL team and only a smattering of concerts and other suitable events for a stadium. The Cogswell interchange therefore might be better repurposed for more intensive economic activities than just a sports stadium - pehaps more high rise development should be considered instead.
Shannon Park - A bad idea all the way around. The only walkable area nearby is Highfield Park (a slum) and major costly infrastructure upgrades would be necessary to make the site accessible for large numbers of fans. Even all the chatter on here about ferries would entail literally tens of millions of dollars to accomplish. Now add on the costs of an interchange to directly access the site from the MacKay Bridge and you would have the Minister of Finance losing whatever of his grey hair that he has left. I also think a location by the water would be cold and damp and the only thing visible as a viewplane from the stadium would be the smokestacks at the NS Power plant.

Finally, although the feds might contribute the land for free, the first nations communities legally have first dibs on it. Be prepared for lawsuits lasting for decades (and making generations of lawyers wealthy) before this land might be available. It is difficult to think of one redeeming feature about Shannon Park.
Dartmouth Crossing - This remains a viable possibility even though it would not be a popular option for the urbanists populating this forum. On the plus side, the road infrastructure around Dartmouth Crossing is pretty well developed and there are lots of options for surface parking in the area. It would be the most convenient location for out of towners coming to games, but the least convenient option for peninsula dwellers. Since the constituency with the most power here are peninsular Haligonians and not Truro residents, I imagine that Dartmouth Crossing will not be considered.
Overall, I think the stadium should go somewhere in the northern part of the peninsula.