From my experience I find Nickels to be the most rewarding, just by generally having the highest amount of pre-2000s coins. You can more easily get young head Elizabeths and 12-sided nickels which are always fun to find. Next I would say dimes for the silver - it doesn't happen often but it's more likely than quarters. I used to like searching quarters but the last couple years have only been good for modern commemoratives. Same goes for loonies and toonies - they're only worth searching for filling holes in circulated sets.
For example my last 10 rolls of nickels was 13% pre-2000s, vs. quarters at just 4%. And I had an unusually good 10 rolls of dimes which was nearly 42% pre-2000s, also which had two silver coins: a 1964 and 1968.
Yes, I might have to agree with you. I've gone through a few boxes of quarters lately and its been sad. Few years ago I did a handful of boxes of nickels and it was fun for a while...but certainly not like it used to be years earlier.