IMO the last car BMW made with a truly great driving experience potential was the 128i.
The N52 came in a lot of specs and chassis-- from 174hp to 268hp, and in chassis from 3050 to 4300 lbs. So, you can come away from it with a lot of varied experiences.
But, in the 128i in particular-- if you add BMW's 3 stage disa (from a higher trim N52), headers, and a tune, you're at 300 factory reliable NA BMW horsepower in a i6 that weights freaking 328 lbs. Almost 150 lbs ligher than an S54! I'd kill for that kind of weight off the nose of my M3!
Combine that engine with a properly optioned 128i, and you have a car that is 3050 lbs stock- and sub 3000 lbs with just runflat delete and light weight wheels.
Add M3 subframe bushings, so the body is actually connected to the suspension, and you have a great driving little car. More factory mods possible if you want more, beyond that-- all the M3 control arms are bolt on, as done on 1M.
128i steering is great as stock-- none of the fake heaviness stuff they did on the e9X.
135i is a 300+ lb penalty over 128i, and all the turbo engine dynamics I don't want.
But, yeah, put a low factory trim N52 (e.g. the 228hp one in the e9X 328i) into a heavy chassis (sadly the e9X is a fairly heavy chassis-- e60 is only 72 lbs heavier optioned like for like), and I could see why you came away not overly impressed.
This is the stock subframe bushings in the e9X non M. They just ruin the handling, especially once you ditch the RFTs that they're there to hide. Replacing them with M3 subframe bushings brings the entire chassis up the the level of non M BMWs before e9X.
I suspect you'd like the N52 manual. Manuals from the same manufacturer generally have shift feel inverse to how much torque they're rated to handle. The Z4M was exceptionally good because they couldn't physically fit the 420g, with its higher torque rating, from the M3 (which was over rated, but does make it nigh indestructible, so... track offs)-- so they used the 330i manual instead. The N52 manuals are similarly low rated, and feel great as a result.
You should drive some more NA M engines
