His first job was taking over a team bottom of the German 2nd division and keeping them up. That's a plus as it shows he had an instant impact (6 wins, 2 draws, 3 losses)
2nd job was Schalke who'd finished 10th the previous year - they finished 2nd in his first season. In his 2nd season, dealing with the added exertions of playing in the CL, they were struggling and he got binned after City webbed them 7-0.
Went to Spartak Moscow - took over a team who were in 12th place on a run of 5 consecutive defeats and got them up to 7th. Following season he got them into 2nd place and CL qualification, he then didn't want to renew his contract
Went to Leipzig after Jesse Marsch got sacked - Leipzig were in 11th place. He got them up to 4th place that season, getting to the EL semi finals and winning the German cup. Got sacked this season after only 5 league games (won 1, drew 2, lost 2) but lost one CL game at home against Shakhtar (4-1) which got him the bullet. Maybe they were right, but 6 games into the season that feels a little bit premature to me.
I can't claim to know everyone who's out there, but this fella has instantly improved every team he's walked into, regardless of their status - he's not wedded to one style or system. He speaks fluent English and he's still only 37. Of the available candidates that we've seen mentioned, he seems to have the most relevant CV for what we need right now.