I will say I am a BMW guy, so there is bias here. But I've had lots of experience with German cars, both owned and friend-owned, and worked on them a lot. (Specifically the 2.7T, 1.8T motors from Audi and the S50/S52 motors from the e36 M3's and the S62 motor from the E39 M5.) The bang for buck and maintenance costs are in my experience lower than Audi or Merc. With that said, German is German, so don't expect Ford or Honda maintenance costs. I've personally never dabbled in Audi or Merc but my friends have, and lets just say they all went back to BMW's. As long as you take care of them, they will treat you right. The guy I sold my e39 M5 to in Mentor is/was selling his E46 M3 because he's DDing the M5.
E36 M3 (may not have enough straight line go for you coming from a bike, car is still fun to drive.)
E39 M5. (You can haul the kids and haul ass. German v8's are intoxicating.)
E46 M3 (Metallic exhaust note, less seating room.)
E46 ZHP (Basically the 4 door e46 m3 that bmw never made.)
E60 M5 (More in maintenance costs, but German v10 sounds amazing.)
E90 M3 (Again, German v8 and comes in a coupe or a sedan.)
Stay away from the audi's the 2.7Ts (aka b5 s4) those can be money pits. The 1.8T's arent quick or fun at all unless you mod them, and then they become not so reliable.
Stay away from Mercedes. AMG's can be fun but on average you'll dump more money into a Merc than a bmw.
I wont even respond to the G37 (slow) or the TT comment (Unless it's a new TTRS).
Tony, you are a smart consumer so I know I don't need to tell you to do a PPI at a 3rd party dealer on whatever you plan on buying. If the current owner or dealer isn't willing to do a PPI, then I'd move on.