For me it's Diablo II. Granted I've played my fair share of D2 since launch, and also recently on a private server with a comrade from hexbear, but I still feel like years later the game didn't grab me as much as D1 did.
Granted I don't hate D2, but for a game that I keep coming back to, D1 takes the prize.
Seriously though a 40K game has the potential to break out of the mire by turning towards the tabletop for inspiration. Imagine if it was kind of like Total War - build and customize your army, have your units gain experience and become elites, have a persistent online league where you can fight for territory with your guild (remember guilds?), etc. Based on how successful Commanders was in Starcraft, I would make a PvE campaign mode the banner feature and advertise it hard to people who like RTS as a concept but don't go in for competitive PvP.