I only have 3 mods I feel are must have regardless of anything else. -B9 Aerospace Pack, which contrary to B9's own statements, does match the current feel of the KSC. It's pretty much just an expansion pack for stock KSP, and in the next update, it should be the top of the tech tree. -Kethane, to be honest, is near 'bout perfect. If Squad today released it as the stock basis for all further resource expansions, I highly, extremely, doubt they could do better. Kethane's maker has hit the nail perfectly. -Kerbal Attachment System, while not as perfect as the other two(look at your detachable fueling port), it's just so good. And like as been said by previous posters, it removes the massive difficulty of docking while landed to refuel. Other than that, there's a few mods that would be nice to see added to the game, but these three? Squad, just add them and be done with it.