OpenTTD for sure :P I guess we could either have weekly mega-sessions or we could have one constantly running one on a massive map which resets after a certain period (maybe a week or a month?), so we can play long-games on it with competing companies
I'd be tempted by a Minecraft server, though I don't actually own a copy, and I reckon Space Engineers would be a lot more fun with a bunch of people and teamspeak.