There really is no "one" best.

And in what regards to you mean "best".

If it's in regards to design break-throughs or genious design...that goes hands down to the A-5
If in regards to aesthetics...that's completely subjective (in fact this whole thread is subjective) I vote Win 21 (that's there old SxS, right?)
If in regards to throw-it-on-the-ground-stomp-on-it-and-slap-till-your-happy dependability SSSSOOO many would score a perfect 10 its not even funny.
If in regards to value/price ratio the 870 express certainly will be hard to beat. Maybe not hard to tie...but to beat. (Lets not forget the whole mess of single shot NEF-styles that are out there.)
If in regards to a choir of angels proclaiming "This is the shotgun, divine and wonderful." It's whichever one you break your first 25/25 with.

I have a Rem 11 (which is really an A-5) 2 870s (12&28) and SKB XL900 auto, and a Savage Fox B (SxS). All function flawlessly except for the SxS (a loose safety tab that likes to bump closed after the first shot) The 870s have the worst triggers and the 12gauge is the heaviest of all my shotguns, the Rem 11 is prone to cracks in the wood and the SKB is hard to find parts for. Every gun has its issues, none are perfect or perfectly suited for all manners of hunting...there will never be a "best".