The big factor that makes it difficult to call, obviously, is the fact that it'd be the Europa League rather than the Champions League.
If it were all else equal and the Champions League instead of the Europa, it'd obviously be the greatest single season in the history of club football. But many will rate it down a bit because it's the Europa League, which isn't entirely unfair in my opinion.
It's also probably fair to say that winning the Bundesliga isn't as impressive as winning the Premier League, but that's not Leverkusen's fault.
In the end I still think our treble is more impressive, given the way we won the CL final and the fact that we won a Premier League that was arguably more impressive than the Bundesliga ever was, and we only lose 3 matches in the league that season.