There are a million answers to this question...LOL. But a Contender/Encore or a good revolver will do the trick. The caliber depends on shot distance and whether you reload or not. A 357 maximum or 445 Supermag may be a great rounds, but your not likely to find these rounds on the shelf of your local sporting goods store. I own 2- 445 Supermags, 1 Contender and 1 Dan Wesson, but I reload and treat the brass like gold, as it is not the easiest to come by. I shoot 44 mags out of them in the off season. I would personally recommend a 44 mag in a 14 inch Contender/Encore or a good revolver(Dan Wesson, S&W, Ruger, etc) with a 6-8 inch barrel. The 44 mag used to be the big dog 5 decades ago, but it is middle of the road now days with all the big cals out now(454 Casull, 480 Ruger, 460 S&W, 500 S&W, etc.) The good thing is there are mild to wild loadings for this caliber which makes it manageable to shoot for most shooters. Another good thing about the 44mag, as long as you make a good shot(shoulder, chest, vitals), it will drop a deer in a heartbeat,
If you can use bottle neck cartridges, there's a bunch of good ones for the Contender/Encore. But for straight walled, the 44 mag is probably still the most popular, and ammo can be found at any place that sells ammo.....