"Best" can mean cheap or easy, depending on your perspective. Easiest is to buy a Moss or Rem with a 14" barrel and change the furniture to what you want. $600 will get you a TAC-14 or Shockwave, $160 for the Magpul SGA Stock, and $45 for the Magpul forend = $805. plus tax (and possibly shipping). You can buy a used Mossberg 500/590 or Remington 870 shotgun and use the receiver for a build. $205. for the Magpul furniture, $300 for a 14" Rem 870 barrel, or $400 for the Mossberg 14" conversion kit and the remainder for the donor receiver ($300 or less for the Remington or $200 or less for the Mossberg - to stay under the $805). For the cheapest, go with all used parts for a build or start with a used TAC-14 or Shockwave plus used furniture.
I prefer Mossberg shotguns, however I went with a Rem 870 receiver for my shorty shotgun. There are simply more parts for the 870's and at the time I couldn't find a decent priced 14" conversion kit for my Mossberg. If I was to do it now, I would buy a shockwave and change the furniture, given Remington's QC shortcomings over the last while (which I hope changes given they are an iconic brand).