What is a good caliber for black bear and whitetail deer? I have a .243 but I am guessing that it is too small for bear.

IMO, your guess is correct. I'd recommend such as the 280Remington, 308Win., and the 30-06. With the heavier bullets available in these three (160gr., 165, 175, 180's) they will handle the bears, and of course, are plenty for any deer.

.270 &30-06 are 2 of the most popular black bear/deer guns

30-30 would work too. 100+ yards for deer, and usually bear are in closer range. I have never hunted bear, however, usually it's done at close range and in the woods around here. It's nice to have a lever gun that you can easily get around and up a tree with.

30.06 or 270 will work for both as well.

A 30-06 will handle both whitetails and black bears perfectly. A great caliber for all north american game.

There are many many good rounds suitable for black bear and deer. IMO they start with the .243 and work up to the .375 H&H.

The .270-.280-.30-06 class are more than powerful enough for the task. My personal preference for this category of game is the .25-06, .260, 7-08 class of cartridges. The .260 and 6.5 X 55 being at the top of my list. The .243 will do the job as well on deer but for a 400 pound black bear I'd like a little more oomph.

7mm-08, 308, 30-06 would be my main considerations.

I guess it all depends on what the conditions are that you will be hunting. How far will the shots be? If you are going to shoot beyond 150 yards, a .270 on up a .375 will be fine. I would probably recommend a .30-06 since factory ammo is easy to find anywhere in the world. If the shots are closer and in thicker areas, don't rule out the .45-70. This caliber gets no love but I really like my Marlin in that caliber. It would have no problem taking down a bear and I don't have to track a deer when it gets hit with a 300 grain bullet. It would also make a great Elk rifle which I plan on hunting next year.

Most of the all around calibers will work well. This would include but not me limited to the 270, 7-08, 280, 7MM Mags, 308, 30-06, & the 300 Mags.

If you aren't a handloader I would choose the 30-06. Ammo is available just about everywhere and there are a plethora of loads available.

7 X 57 and 8 X 57 are also good choices. If you habdload you have many options.