I collect few series:
1. Central American Republic. Likely the most beautiful coins ever minted, IMO. The Sun shinning, the mountains, the three. Just a great design. And they are rare.
2. Mexico's Iturbide. A very interesting historical period. Misunderstood by many. And very rare issues. Despite the few coins in the set, it is likely impossible to complete (try finding a KM305 or 311). And his proclamation medals are both beautiful and rare, so a challenge.
3. Mexican republic gold. I just like the design. Particularly, the 2 and 4 escudos.
4. Mexican peso fuerte. An amazing series, less popular than 8 reales but very challenging.
5. Mexican Ferdinand VII half escudos. Short lived, incredibly difficult. This I hope to complete one day. As well as a high quality Caballito Mexican peso. These 2 series are the only ones I think will get complete as they are just a few coins.
6. If I ever get the money, perhaps I will branch to Chilean 8 escudos (book and hand). Not as pretty as their Mexican counterparts IMO but very beautiful in their own right.
I have never been interested on completing these sets ($$$), but I enjoy collecting them. Great coins with fantastic stories and histories behind them. I have been always more interested on quality and rarity than quantity. I think that is why I will not complete these series.