Guardiola is a good coach, there's no question. I'm very excited to see him with a club like Man City though because so far Guardiola has only worked with the absolute best players the world has to offer.
Even coaching a player like Aguero seems like a step down in the quality of player he's used to. I can't say he's the best coach in the world until I see him achieve something with a lesser side. His last two defeats in the CL should be telling. It's something like 11-2 on aggregate to Real and Barca if I remember correctly.
It's one thing to employ your trademark style when you have optimal conditions. But coaching is not always about that. It's about adapting and pulling something out of nothing. Fans always groan about counter attacking football but it's a legitimate philosophy that must be coached. If you look at someone like Simeone, his team can counter attack and it can play possession, this is why he can keep up with the giant teams year in and year out.
He can't simply rely on his players to execute every match, because they won't. They're not as good as Guardiola's. So he has to adapt the team constantly. Guardiola has never needed to do these things.
Can you imagine him at Man City and Aguero gets hurt. Here you go Pep, go win the treble with Bony. And it's possible Guardiola is so great a coach that he can do something with that caliber of player but to be honest, we haven't seen it yet.