When you're new, your
best bet is either Cents or Nickels as you'll
always find something interesting, old, or odd.
Nickels are my
highest recommendation for your first run as you'll find at least one
War Nickel, Buffalo, or something special per box, as well as a good deal of pre-1960s coinage.
Cents come in a close second because of all of the variety. You should find about a dozen wheats per box, and there are lots of varieties. I also recommend having a magnifying glass on hand as some of the fun stuff to rake for in cents requires an extra close look. :-)
Dimes, Quarters, and Halves tend to be
very picked-over for older coins due to silver-searching. However, I would recommend them in this order:
Third place goes to Quarters, and I recommend that you grab a
State Quarters album before you nab a box, as you'll have nearly every hole filled when you're done. There's a lovely variety of reverse designs to enjoy, and you have a 1 in 4-ish chance (per box) of scoring a silver quarter.
Fourth goes to Halves, strictly because of silver hunting, as the "payload" tends to be the "largest" when you
do get a payload (and there's just something about the feel and sound of a silver half dollar when you flip it that is
so satisfying :-) ). The biggest drawback is if you hit someone else's culls -- and you'll see it as they're all marked -- or get hit with a skunk (i.e. no silver) streak it tends to be a morale breaker.
Fifth goes to Dimes. Nothing very interesting happens with them except for the once-in-a-blue-moon
Mercury dime (1 per every 4-5 boxes), 1982 no-mintmark, or just as rarely a doubled die (but there are so few DDs in this series that are easy to spot it's laborious). You might get 1 or 2 silver dimes per box, if that.