Halves, even though the odds of finding something good have taken a hit thanks to a barrage of coins issued recently for circulation. CWRs in particular tend to bring better results. I like to spend many of them as well since I tend to be more of a "small time" searcher with different hunting priorities for other things.
Second is nickels, due to having the highest find rate of pre-1960 coinage of any denomination while providing the possibilities of better grades, semi-keys, WWII silver, Buffaloes (I found a Hobo once), and for a few lucky searchers, the odd
V nickel. The profits though for most of the common 1939-1959 coins in less than EF condition were small enough though that I resold many of them years ago following my peak searching years.
Third is dollars. Sure, most of what's out there are small dollar rolls, but the odds of finding NIFCs, proofs, and 1979-P wide rims are quite high compared to the other denominations. The gravy is finding the large, "crown-sized" dollars. Ikes are scarce enough, but finding a 40% silver Ike, a 1921-P Morgan as well as a 1922-P and 1927-P Peace were definitely moments that I'll treasure (about once per decade finds). Thanks to inflation, they're more accessible to search at a time a single dollar barely gets you a pack of gum or a small coffee at McDonald's.
Fourth would be cents, mostly due to fond memories of hunting them, where I've found several thousand Wheats including several 1909-V.D.B.'s, a 1910-S, and a 1918-D in EF, about 20 Indians including an 1869 and a beat-up 1879, and even a few errors. They're still not worth much, but the copper content has played a role in the rise in a typical bag of 5000 Wheats up to around the 6-7 cent range.
Even though I tend to have a better silver find rate than quarters and have my oldest ever silver find, a 1907 Barber, dimes don't appeal to me too much, mostly due to their smaller size making it not as easy on the eyes to search. Since I don't care much for varieties, most of my hunting comes from edge searching, and it seems that half of the time, the white edge turns out to be either a Canadian dime or British 5p coin.
Despite the appeal for W's, I'm just not much into modern quarters ever since the USM expanded well beyond just the state/territory series and into the NPs and women series. I've tended to reap the profits from my half finds (the 40% silvers while I've kept all 90%'s) into buying quarters, and that has eventually led to a complete 1932-64 Washington set.