If you only want to buy individual packs, then your options will be quite limited as most places will only have retail product boxes opened.
10/11 Donruss, 11/12 Hoops are just a couple I can think of that you may find as loose packs?
As far as the 'best' set?? It depends what you want/like. There are a lot of different releases around, so it would probably be best to have a look on Youtube and see which releases you like the look of and start collecting that?
Also, decide whether you want to collect base or inserts or Jersey or Auto cards!