it depends on the key of the song you're planning to play. but C and G are great keys to have. i use those quite often. D comes in handy often too. if you don't have the right harmonica to match, you can always transpose the song to another key.
If I could only have one harp it would be a C. That gives you straight C for tunes in C Maj (folk and similar styles) and also G in second position or "cross harp" for blues in G.
As it is I have harps in a load of keys I never use. I reckon my C and G ones are the only ones I've played in the last 12 months.