I rocked the Stag and the River as my main fairway drivers for about a year. They make a great combo. The Saint would be a tick less stable than the Stag, and the Saint Pro noticeably more stable than the Stag. Personally I think the Stag is the best complement because it's in between the the other two that were mentioned. A max weight VIP Stag, a 160s Tournament Stag, and a River can cover a lot of lines together. Especially as the Tourney Stag wears.
The big advantage with the Saint Pro is that it's way better in the wind. It's a lot less versatile for shaping lines than the Stag, though, especially if you don't have a big arm. It depends what you're looking for.
I tried the Eagle, but it wants a sturdier throw, so switching between it and the River was a bit awkward. The Stag likes a smooth throwing style, like the River, so that helps in moving from one to the other.