If you stay in or near Dolgellau or Machynlleth then Nant yr Arian, Cli-MachX, Coed y Brenin, Penmachno and Gwydyr/Betws y Coed are all close enough for driving to. Personally I prefer Machynlleth to Dolgellau as a place to stay, but Dolgellau is a bit more central to the various trail centres.
Edit – Of the above, my favourites are Coed y Brenin and Nant yr Arian. Coed y Brenin is fairly rocky and has the most trails. Nant yr Arian has a more back country feel and lots of lovely swoopy singletrack. Cli-MachX is mainly about its big final descent. Penmachno is fairly old-school XC except for its big finish. I can’t remember much about Gwydyr, it’s been 7 years since I last rode there.