Depends what you want to do with it. I have 4 of them, two in .177 for 25M benchrest and FT. Unless you intend to shoot long range or to hunt, the larger calibers are nice to have, but more expensive to shoot. I have a .22 and a .25 which I can still use in 25 and 50M benchrest. I had a .30 and it was a beautiful rifle, but I had really no application for it, so some other lucky individual is enjoying it now.
 
.177: close range, small pest control... field target competition.... 
.22: if you use heavy pellets you can achieve long ranges and take down medium game at close range or small game at long ranges.... 
.25: for me is the most versatile caliber..... you can bring power down and hunt small game at close ranges and gain extra shot count... or you can get the polygonal barrel and bring power up and shoot long ranges with enough power...
.30: its made for long ranges purposes....