Powder, from any company that you find reputable. I like the micronized form. I would avoid any of the gummies, as a huge percentage of them have nowhere near the amount of creatine they claim (I believe the cooking process denatures it or something but this is just conjecture).
I get mine from bulksupplements. Gummies and many gym focused powders are extremely overpriced. Buying in bulk you should be able to get it for ~$.15 a day at 5g.