The effectiveness of the ranger is constrained by their favored enemy. Granting rangers favored terrain did little to help this. Not fighting dragons? Too bad. In snow? You're out of luck. I do think quarry was a very good step in the right direction but I think it falls a little short and it's benefits come a little too late.
Here's my suggestion: change the damage bonus granted by favored enemy to 1d6 per + (for example, a 5th level ranger deals a maximum of 4d6 damage to one favored enemy), then make quarry so that a ranger can apply half his/her favored enemy bonuses (which are all multiples of 2, so this would be very easy) to a single target after studying it for 1 round (maybe increase this to 3 rounds to put on par with the assassin, or keep as a standard action if you feel generous). I'd also bump up when the ranger gets quarry to early levels (maybe 2nd or 4th level).
I think that this would really allow rangers to maintain their effectiveness while not being confronted by their favored enemy and emphasize their role as a surgical striker and marksman.