How can I choose the model that best fit the equations?

I have the following equations: Linear : 1.90x+16.87; Quadratic: 0.04x^2+1.59x+17.31; Quartic: -0.01x^4+0.25x^3-1.38x^2+4.69x+15.26 Which one fits best to these data: (1,18.8) (2, 20.9) (3,22.4) (5, 26.1) (6,28.3) (7,30.4). Should I disregard the quartic model since it starts with a negative number?