Best card to boost credit?
Congrats on getting off to a good start. The most important thing you could do to help your credit would be to read the posts here and learn the woes caused by even minor credit mistakes.
To answer your q. though, get a store charge card (Macy's, Home Depot, etc.) from a place you would shop a couple of times a year anyway. FICO likes to see a mix of credit types. I'm not sure if FICO counts store branded Visa/MC as store or regular CC's because you can use them anywhere.
I may be zero but I'm not worthless!
TU Fico - 565 > 737
EQ Fico 561 > 741
EX Fico 560 > 738