You'd certainly have some Sense.

Obviously the easy answer is to pick one of the Huge materia or even the Enemy Skill materia. If you exclude those, I'd have to think a little more carefully... I mean, I don't need to go around killing anyone with massive attacks, so I'd have to be a little more inventive.

1. Carbuncle, so when it rains I can just have the rain bounce off me. Also, he's pretty neat.
2. Ifrit, for hanging out on cold days.
3. Shiva, for hanging out on hot days.
4. All, so I could sell it for a large sum of money.
5. Chocobo Lure, so I could lure chocobos!
6. Manipulate, so I could use people as my puppets.
7. Luck Plus. Who couldn't do with a little more luck?
8. Gil Plus. Earn twice as much at work.
9. EXP Plus. Gain twice as much knowledge from my experiences.
10. Pre-Emptive. Always be one step ahead of the game.
11. Underwater. CAN BREATHE UNDERWATER. x_x
12. Time. So I could do everything twice as fast. Stop could be useful, too, when people are annoying me.
13. Full Cure. So I could instantly repair any non-fatal injuries.
14. Esuna. Never be sick again.

If I had all of these things I could basically live happily forever. Picking just one is really rough. I think I'd go for Esuna, Time or Gil Plus. I mean, they all relate to each other in some way. With Time I could earn the money of two men by doing everything twice as fast, and I could arguably Stop any illness in it's tracks if I could control the spell well enough. Earning more money would also mean I could afford excellent healthcare. But then, Esuna would allow me to be healthier for longer, therefore live longer, and let's not kid ourselvse, Esuna is a proper money maker. The real concern would probably be the high demand for my services should I have Esuna. EXP Plus would be neat, too, but I think I'm rather content with the amount of experience I'm getting out of life right now. I just want more moneh so I can have the opportunities to gain more interesting experiences by traveling etc.

I think in the end I'd probably go for Gil Plus. That double-your-salary deal would set me up to be able to do a massive amount of things I really want to do in life.