Personally, I hunt mostly nickels and halves.
Nickels usually yield at least 1 silver out of a box and even if it doesn't, you will surely get 20-30 pre-1950 coins. I also usually get at least 1 from the 30's in every box. I have found a couple buffalo's as well, and I have a couple friends who have also found some buffalo's. They are out there.
I focus on halves at Christmas, many do not know about the 40% silvers so you can get some of those sometimes, and you can get lucky and get some 90% as well. Plus, there are some other halves that are worth more than face value. 1987, 2002-present are all not intended for circulation and were released for collectors in mint sets only. "S" mint mark coins as well are found sometimes.
I dabble in all denominations and I agree with boomer, you are not going to find many silver quarters. I have only ever found 1.
Dimes in my area do not produce silver very often, but i hear other areas of the country get a few out of every box.
Pennies get me between 10-20 wheat pennies per box, but I have never found an Indian (oldest was a 1919 wheat)
This is all subjective, you will have to dabble and find out what you like, but after searching hundreds of boxes, I'm a nickle/Half guy followed by pennies, then dimes, then quarters.