Tbh any Bundesliga club would be fun. I'm loving my Dortmund save but can be pretty easy so putting rules/restrictions can keep the league competitive and make it more enjoyable. (eg. Academy Homegrown players, only sign certain nationality etc.)
But, I agree with askep. Leverkusen is the best bet. Some good players with potential and as askep stated you have much to achieve. In my save they won the league 3 seasons in, the cup and the Europa Cup. It was an amazing season for them, Schürrle is a god in my save and still there.
One of my best saves ever, was with Hoffenheim. It was when Ba was there though. Good side (and I finished 3rd in my first season, there highest finish ever). For some reason, I quit them for Buyern an it was a terrible mistake and I ended the game soon after.
i'm playing as schalke now. i have been for the past month. it's very fun
raul+huntelaar make a very good tag team. they're 2nd and 3rd highest scorer in the league now for me (podolski's first)
schalke is fun AND challenging that's why i like it so much. most other teams are either fun OR challenging. schalke in FM are stronger than most other german teams but you have to smart with your finances. schalke have the second highest wage bill in the league in FM12, so you really need to watch your finances and make smart transfers. i turned off "first transfer window transfers" so because i couldn't offload any players, i was stuck losing €4m/month until december. i'm not even exaggerating. losing €4 million/month. i pretty much have to rebuild the squad with "cheaper" players over the next few transfer windows with a long term goal of increasing the club reputation so i can increase my TV revenue. like i said, you have to be very financially-orientated to manage schalke
i also managed dortmund but it wasn't fun. i quit after 1/2 a season. i never tried leverkusen but they have boring players IMO :P
Somewhat irrelevant as my suggestion is neither of your trio, but RB Leipzig are a fantastic save in Germany. You have to holiday the first season for them to be promoted into the 3rd division, but they have a good enough squad to get promotions to the Bundesliga, and they are backed by Red Bull.
Ah the Bundesliga. My nemesis! I have never won this league in several attempts lol. One day I will though, it's one of the best, if not the best league to manage in. Certainly in Europe anyway. I've had success in promoting teams but just can't crack the top division nut!