I highly recommend Brazilian Jiu Jitsu - It's an incredibly technical martial art that is equal parts physical and mental. For every move, there's one or more counter-moves. For every counter, there's one or more counter-counters. You need to be thinking about what you can do, what your opponent can do to stop you, and what you can do to stop him from stopping you all at the same time. The difference between success and failure is often your knee/hips/arms/centre being 1cm off. Additionally, there is no single "standard" style. Every person has their own unique way of doing it, which keeps it quite interesting.
Furthermore, your strength advantage will be somewhat negated by the fact that it is so technical and if you do decide to compete, weight divisions almost entirely negate any size advantage you may have.
As an aside, I'm calling bullshit on anyone claiming that "X martial art completely ignores the size of a person". It's total bullshit. Yes, a 5ft nothing woman could beat the shit out of a 6ft guy if she's a black belt and he's clueless. But equal skill level? Big guys wins 9 times out of 10.