Kterou z těchto sušiček chcete?
I put the same sentence without the "ty". Is the "ty" here just to stress that it's "you", or does it have some specific other role?
why isn't it "Kterou jedna z ..."?
You can't translate every word when learning foreign languages. English "which one" is just "který" in Czech.
The English would also be perfectly correct without the 'one'.
Chceš is already in 2nd form, you dont really need tý in the end