If you're looking for the cheapest to get grandfathered on the unlimited data plan, it's the vortex. As far as the "best" it depends what you want in a phone.
You can get some certified ore owned phones pretty cheap. I think last I saw the D2G was $99 with a 2 yr agreement. Which I thought was a steal since I payed $299 for mine new. Check out verizons website they may have other phones cheaper now.
I picked up an original Droid off Craigslist for $50. Been working for a while now.
Shop around CL or Ebay and you will find something. Only issue may be, that right now, sellers know you want it badly so you can use it for the grandfathering into UL data.
Still a good deal.
I paid $45 for my original Droid brand new from VZW almost a year and a half ago. Still working well. You will love its unlocked capabilities.