1-Modern Diplomacy: Best variant in terms of map, balance, and size, but has the Build in Any Supply Center You Own, or “Build Anywhere”, rule, which is a must (shouldn’t be too difficult to code though) — and a great rule for many variants (for the future)
2-Classic Crowded: Super cutthroat, and also fun-sized
3-Fantasy World: Nice to be out of Europe sometimes, and the two-island concept is fun. And also nicely on the larger size, but not so large that players are more inclined to miss moves, or that it becomes impersonal
There are a of great variants with one or two almost standard special rules, like Build Anywhere, having standing neutral armies that disband when attacked with support, or being able to transform unit types when on a supply center, so you are really limited