Just as an example of why I think Bruce Sherman, Derek Jeter, and Mike Hill are doing the right thing, Fan Graphs has re-evaluated the Marlins player development organization:
https://www.fangraphs.com/blogs/top-...miami-marlins/
So to answer Bobble's question and bring the conversation back on topic (since I'm the one that derailed it) The Marlins are not really that big a challenge. There will be no joy this year but if you are their GM and make a good move with Starlin Castro and JT Realmuto this season the future is bright.
I think the hardest team to manage would be the Tampa Bay Rays. You would be revenue challenged with a team that is good but not playoff good.There are nice pieces in AA & AAA but as soon as you bring them up you have to sell a veteran you won't be able to resign. It's like running on a treadmill. You are busting your butt but never getting anywhere.
Edit: Or the Seattle Mariners. Their window is closing and they will soon be in a rebuild. It would be a challenge to take a team through a rebuild over a few season and see what you can make of it. Not a team that is coming out of one like the White Sox, Padres, Braves, Phillies, etc.