I guess it depends on what your interests are, but I have always found the Spanish Colonial pieces to be very compelling and historically interesting. In particular, the large silver 8 Reales struck around the mid 1700s through the mid 1800s are interesting -- as they were legal tender in the USA until 1857. Some can run a bit more pricey (I have a couple in the $400-$500 range; you can find MS pieces in the $2K-$3K range), but you can also find well worn but nicely detailed examples for around $150 or so. I have an example minted in Peru in 1806 graded VF by NGC that is just a joy to look at in hand.
They show up for sale on Heritage in about 2-3 of their annual large auctions, and occasionally in their weekly auctions that end on Thursdays. They are also frequently available on eBay, but you have to be patient for a nice piece, and I'd stay away from the "shipwreck" examples with pitted surfaces and damage. I only own three pieces, but I found two on eBay and one on another coin forum buy/sell/trade. Do a Google search, and a little research online and you will be able to find lots more information. I am by no means an "authority" on the series, but they are some of my favorite large silver coins. I'll leave you with my images of my favorite example -- an 8 Reales minted in Lima Peru in 1793 with some lovely toning and a great amount of detail.
Cheers,
-Brandon