I just went through the same adventure. You can read about in in the Optics section of the forum.
Short answer - if you're going cheap and don't mind a thick reticle, the UTG or Leapers Bug buster is OK.
I'm fussy, so I ponied up for a Leupold VX-1 Rimfire 2-7x28mm. It fits the bag and is VERY nice.