It's incredibly hard to rank IMO. Do you rank it as just a single season? Or does the team have to have repeat success to be ranked higher? Do you factor in playstyle? Number of wins? Number of losses? All competitions? Just Premier League?
If you want to factor in everything, then honestly, the only correct answer for no.1 is United's treble winning side. It isn't my favourite side, as I think our 07/08 side was better personally. Arsenal going undefeated was amazing of course, but they didn't win the Champion's League or League Cup. Similar goes for Man City's 100 Points or winning all the domestic trophies. They both had more losses in all competitions than United's treble side. In fact, Man Utd's treble side played more games than any other side ever has when counting all competitions and lost fewer than any other as well! Yes, that includes all the teams in the OP and may even include Liverpool this season (United's treble side lost four games in all competitions that season, Liverpool have already lost three with a minimum of 13 games left to play, technically they could go out of the FA Cup and Champion's League without actually losing another fixture mind).
It's easy to overlook United's treble side despite the trophies as the Premier League points tally was so low, but realistically winning the three biggest trophies and only losing four(five) games out of 62(63)*
*(If we count the Community Sheild [then Charity Shield])
Still, opinions, everyone has one. I will say, if Liverpool somehow go on to win the FA Cup and Champion's League this season, whilst maybe only losing one or two more games overall, they're probably the best in a single season in the Premier League era surely? Long way to go though. Of course, they'd have to retain the Premier League title at least once as well to be classed as the true greats they probably deserve to be over a period of a few seasons and not just more of a one-season wonder, which is what I think is a reasonably fair assessment of Arsenal's invincible side.