As mentioned Final Fantasy XIII's battle system is an evolution of Final Fantasy X-2's battle system so if you want to go with pure combat that might be a good choice, though chances are you probably won't like the story. If you go with X-2 grab X as it's the prequel to X-2. Combat in Final Fantasy X is probably the most strategic of the 13 main titles, though the combat's rarely ever utilized cause the game's too easy. Final Fantasy 1-9 are essentially the same gameplay wise, especially the first 4; standard turn base affair, though he series doesn't really try to tell a story until Final Fantasy IV. To sum up I say start with IV and work your way up. If you like and get a feel of the games then grab the first 3 for kicks and see how the game's progressed since the beginning.
