About the best I've found are the Leupold 2.5X-8X. Plenty of magnification, plusa good compromise as to size & weight...............

Parallax adjustment is not the same as image focus. A 3-9 X scope set on 9X looking @ 300 yards will appear as 33 yards. Even I can hit a 33 yard shot (with iron sights even). Go with the 4-12 if you want but not me... a 9X is plenty of magnification for most common situations.

Semi - I would also look into the Burris Fullfield II line. I have 3 of them in the 3 -9x and they are nice and clear and the ballistic plex works great.
I also just purchased a Nikon Prostaff 2-7x for my muzzleloader. It has the BDC reticle and is also clear and sharp.
For the use you plan on giving your scope, I think the 3-9 will serve you well.

At 300 yards you will not notice the difference between 9X and 12X when trying to judge animals. In order to judge animals the quality of the glass is what comes into play, not magnification. If there is nothing wrong(mechanically) with your scope then I would keep it. You would be far better served by buying good quality binoculars. Spend as much as you can. High end 8X binoculars can watch a deer bat it's eye at 300 yards. Tom.

I'm also a fan of 4-12x magnification. If that's what you want go for it. Everything is a trade off like others have said. You could also do what Tom suggests and get a good set of binoculars and then you won't need as much power on your rifle. I have both.

Tom