First bbl?....
Take straight to the range and shoot, and teach kids or others to shoot: 22lr
If you are someone who likes to tinker / reload / plink - 22 hornet.
If you hunt, and reload - 7-30 waters
If you hunt, and don't reload - 30/30 rem
If you think you might want to compete - 7TCU
Read and review this bulletin board. Check out Eds TCs. Get an idea of what is a fair price for a used bbl. Buy used at the right time, and you can work your way through quite a few calibers.
Watch out - they are habit forming
Nate