Depends on your definition of best and the conditions surrounding each league.
In terms of physical input - strength, speed, energy, tackles, tempo - the premier league is the best. I've watched quite a lot of games in other leagues and been bored by how slow everything is.
In terms of technique - passing ability, touches, composure, set piece delivery, long range shots - I think the premier league is quite far behind. When you're expected to play at 100 mph for a full game, it's very difficult to be technically good too.
As for English teams doing poor in the CL I think this is a poor measurement of quality. Outside factors have a big impact, like lack of winter break, fixture congestion, pace of the games, number of pre-season friendlies, pre-season travel time, fatigue and more. All of these make the PL teams go into tournaments in a poor way which doesn't truly represent their ability.
When we played in the CL we were utter shit compared to what we were capable of. In terms of quality, we'd finish top of that group with some ease. Yet we didn't and this represents England in the CL really. We perform well below expectation, not because the quality isn't there but due to outside factors.
If City gave up on the league and focused on the CL, I think they'd get into the last 4. But they won't, and so they don't. Just like us in the Europa League. If we put our all into it, we'd go very far. But we rest players or we sub them off quite quickly for the game in the weekend.
PL teams don't see the CL and EL as precious and valuable as other European clubs do, so they never invest 100%.