They're certainly one of the best. Especially over the past two seasons.
I don't know if they're all time best, but at least over the Premier League Era, they're probably in the discussion.
They're just so consistent and have so much depth. They've also not really been at their peak pomp and power, largely due to the injuries they've weathered. I mean, they've been basically playing their third choice fullback for a while now, along with only having KdB and Fernadinho for some of the campaign. It's pretty disgusting when they can bring on Jesus and Sane for kicks in the second half most matches.
Now, that said, they still need to cycle out some players. They went away from using Stones, Kompany's old, Aguero turns 31 this summer and he's never been Pep's favorite, and some of their longer term injury concerns need to be sorted out. For all their abilities, they do still use a lot of David Silva, who's also pretty old at this stage. They have the money, the time, and recruitment, where it won't be the biggest of issues, but there's just as much of a chance that they take a step back in getting in new blood.