How are you judging "the best roster", on raw "talent", potential, drawing power, proven success?
Rosters at different times have played to different strengths so it's hard to say one was definitely "better".
Take August 1991 - That roster was arguably the strongest of all time in it's mixture of all the above.
Main Event
Hogan, Warrior, Savage, Slaughter, Iron Sheik (4 of the last 5 WWF Champs)
Ric Flair (Former NWA World Champ)
Sid Justice & The Undertaker
Ted DiBiase
Midcard
Ricky Steamboat, Kerry Von Erich (Former NWA World Champions)
Rick Martel, Curt Hennig (Former AWA World Champions)
Bret Hart
Davey Boy Smith
Jake Roberts
Jacques Rougeau
Big Bossman
Legion of Doom (former NWA and AWA World Champs)
Nasty Boys
Mike Rotunda/IRS
Tito Santana (former IC Champ)
Jim Duggan
Lower Mid
The Warlord
The Barbarian
Skinner/Steve Keirn (Multi time NWA Tag Champ)
The Rockers (Former AWA Tag Champs)
The Bushwhackers
Virgil
Power & Glory
Jobbers/Operners
Brooklyn Brawler
Jim Powers
Koko B. Ware
Orient Express/Bad Company (Former AWA World Tag Champs)
Big Bully Busick
John Nord/The Berserker
Tugboat
Beverly Brothers/Wrecking Crew (Former AWA World Tag Champs)
Throughout that roster there was proven, successful talent that had drawn in their own areas. Even a team like Paul Diamond/Kato and Tanaka, while jobbers in the WWF were in fact a storied, well drawing tag team in their previous incarnation and it showed in their work. They were bottom of the card, but never acted like it. You had hungry new talent embarking on their first national pushes like The Undertaker, Bret Hart, Davey Boy and Sid and proven, bankable talent throughout the midcard and bolstering the main event scene. Sheik was horrid in the ring by then but his role was important, like wise someone like Ric Flair was coming into a place where several of his best known opponents from the NWA were...Steamboat, Piper, Von Erich.
A lot of that talent started to disappear after Summerslam that year... some like Warrior through more sinister reasons but guys like Greg Valentine, the Barbarian and Jimmy Snuka were jettisoned as they had outlived their purpose and new talent came in to replace them...indeed one year later Razor Ramon, Yokozuna, Owen Hart, Tatanka, Crush and Papa Shango had taken their place.
The difference with today or 2015's Roster is more that all the "success" is WWE created except for these guys coming in now...this last crop like Steen/KENTA and Devitt are going to be the very last to have major "achievements" outside of a WWE system. List out that roster like I have above and only a few will stand out as having unique or "real" achievments... there rest will be stupidly high numbers of WWE title reigns etc... There are no actual former WCW champions left other than potentially Sting so it's hard to judge today's crop against the 1991 crop... guys like Ziggler have been a champ but didn't draw to the level even a Kerry Von Erich did with his 3 week reign and all of Orton's 12 reigns don't mean as much as Ricky Steamboats one in terms of real merit or experience... it just shows guys know how to play the WWE game better than others.
That 1991 roster needs far more credit than today's... it was STACKED with real talent, it drew and it was memorable... even the lowest guys had a feud...even someone like The Rockers who never actually "made it" were memorable. Look at those names again and how many are now the legends that surpassed the guys at the top... Bret, Shawn, Taker... that roster was loaded with potential and achieved it... BUT it set the template and WWE has nearly to a man, slavishly followed that ever since, which condemns so many talents to faliure cos they didn't get over cos of that... they made it cos of an enforced change and working with all that great talent above them... Shawn learned from losing to Flair all those times as half of the Rockers...
Austin and the like only made it cos of another enforced change when the company was headed to the wall...
You'd like to think the new recruits will get the same chance... but they won't... WWE isn't capable of it unless they are dragged kicking and screaming, in this case either by the SEC investigation taking Vince out the picture or fate intervening. They have the tools now and the talent... not sure they have the will to let this be the best roster ever.