Strategus wrote:start with one that just does the basics.
It's possible that I'm thinking about the problem wrong from a programming perspective (coding is not my strongest skill), but in my mind the issue is that "best moves" aren't a well-defined concept in Diplomacy.
The reason is that the best tactical moves are almost always conditioned on who is friendly toward you and who isn't, and in particular whether moves can be coordinated among allies. A gunboat AI probably would not have to learn concepts like allies giving each other centers for tactical gain, but it would have to learn when a move is good because of the likelihood of another country supporting it, and inversely when not to play a move because of the unlikelihood of having enough support.
Another super-massive problem for a Diplomacy AI (that chess AIs do not have, incidentally) is when the AI should accept (or perhaps even propose) a draw.