In all honestly, I think that neither of those teams was better than Liverpool. I think that the United 2007-2008 (or even 2008-2009) would be very competitive against this Liverpool and it could have gone either way, the same for City's 100 point side. I think that Utd treble winners and Arsenal invincibles were nowhere as good.
Of course, the United 99 is greater than this Liverpool side, the same as how Pepe is a greater player than C. Ronaldo because after all, achievements matter. But neither United 99 was better than current Liverpool, nor Pele was a better player than Ronaldo. At the same time, the achievements are not the only thing that matters, Chelsea 2012 is not one of the top 12 English sides of all time just because they are one of the 12 English teams that became European champions. Context really matters!