That question is complicated and depends on what kind of fight do you want to have, I have some suggestions assuming you like WWII or WWIII style fights:
1) Continents 60%, 7-9 AI. In this setting each continent is large enough to make it harder to unify for anyone. The continents have wide and narrow areas that enable long borders and sneak attacks from unexpected directions until rails. Also for domination/conquest one needs invading a very large continent with epic d-day battle even on Monarch. Also more land and accessible sea makes higher AI commerce and teching which leads to WWII style invasions possible for you. AI tech rate also increases because of the trade opportunities. They won't invade you but they can gain footholds on AI controlled parts and advance from there.
2) Archipelago 60%, 7-9 AI. On this setting the islands tend to join together to create complex shaped mega-continent. AI still gain communication to each other fast enough. You have land frontage and because this is Arch setting the AI will tend to land troops behind the frontline. Don't leave cores undefended until rails because while your army is at the front someone will invade the rear like the Korean war Inchon beachhead. Also isthmuses can form and the AI is actually building forts and barricades on those.
Climate is also important. The better the climate the more advanced the AI will be. Tundra is worthless terrain and it should be eliminated by using warm setting. Tundra also have oil and hills in polar regions have valuable resources which compels the AI to settle there during REX phase and so waste settlers on crappy useless cities (The AI attempts to grab oil and Aluminum during ancient age despite it will take hundreds of turns before those will become important). Wet setting tends to create too much Jungles and Marshes which will be cleared only after eternity had passed so better to use Arid or the middle setting. The vast deserts of Arid provide good no-mans lands for maneuver with occasional rivers for strategic cities. Age is better to be set to 4bn years as AI likes to use hills for cover, 3bn creates uninhabitable mountain blocks and too many volcanoes that raze AI cities (AI is stupid enough to build cities near volcanoes) 5bn creates too open terrain which handicaps any horseless civ too much.
In general, the more terrain , the better it's quality and the more AI players the better as they will have more units and better tech pace. Only those settings enable the AI to reach tanks and aircraft on Monarch at reasonable date as Industrial age techs are too expensive for a single AI to research in reasonable time. I had seen a tendency for the AI to get stuck in Industrial when there is bad terrain and small amount of land. That happened on most 80% pang and cont maps where the AI gets tanks only around 1850-1900AD and many times fails to reach Modern age at all.
Tech paces for AI, I had observed on Monarch.
Bad setting (Cold, Arid, 3bn 80% Continents, 5AI): Medieval 100-400 AD, Industrial 1350-1600AD, Modern 1950AD-never.
Best setting (Warm, Arid, 5bn 60% Continents, 7AI): Medieval 0-200AD, Industrial 1000-1300AD, Modern 1750-1850AD.
