I will go for the 1909-S VDB cent. I've been wanting it most of my life but I can't seem to pull the trigger on a couple thousand dollars to buy a reasonably good one.
Mercury Dimes, Walking Liberty Halves, some of the Silver commems, the Texas Commemorative in particular, ASE's, the 1987 $5 Gold Commemorative, and a whole bunch more. Hard to narrow down a century of awesome coins to just a handful.
Do they have to be American? If not, many of the British pre decimal coins are awesome when you can get them in the right condition. As for US pieces, MERCURY DIMES ALL THE WAY!!!! Walking liberty halves come in second
Washington quarters are underrated, the reverse is one of the best ever. I like Ikes too, what other coin has a darned Eagle landing on the moon on it!! I also heavily stack Sac's, I like to fill jars full of the shiniest ones. They are my pirates gold!!
I've always liked the $10 Indian gold coin & the 50-cent Oregon Commemorative. If you allow coins of the World, then I would nominate some coins from Italy.
Buffalo nickel for sure, but the standing liberty quarter is also in the running. My least favorite? All the coins with ex-presidents. Bring back the liberty designs.