I'm going to suggest the Mamiya RB67. Bodies and lenses are dirt cheap, quality is along the lines of the best out there (some will say Mamiya is the best out there), very versatile system, and all sorts of options/accessories are available, allowing you to build the system.
From a 'bang for the buck', I don't think there is anything else that gives as much as the RB67 gives at the same price point.
But, as the others have said, it will all depend on what format you like best, and what type of system you prefer.
I have and use 6x6 TLRs (C330 & 124G), 6x4.5 (Mamiya M645), and 6x7 (RB67). On all three formats I have almost everything available as far as lenses, backs, inserts, finders, winders, etc., and I most frequently use either the M645 or the C330. The RB is for 'special' occasions, as the size of the system can be daunting. The RB with 2-3 backs, and 2-3 lenses can be monstrous in size and weight, but the huge negative makes it worth it. When I'm out just photographing, I'll grab the C330 first, and then the 645. The 124G hasn't seen much use lately. It's a great camera, but I like the option to change lenses with the C330.
Just my 2¢ worth.