I recommend Ultimate General for two reasons:
1) Ultimate General is a finished product. Grand Tactician is still in development.
2) Grand Tactician is priced at $39.99 and Ultimate General is on sale for $11.99 right now at Steam.
Sorry I missed your initial post, Nick. My all-time favorite turn-based PC game is Master of Magic , which you can pick up cheap from GOG.com, especially when it's on sale. It's a classic 4X game you might want to give a try if you haven't already played it. I have purchased a number of AGEOD games ...