My thoughts are that the E34 is the last of the good BMWs. They are "user friendly", meaning we can do a lot of work on them ourselves (thanks to this board). For the more experienced, you can do everything on them.
The newer versions have too many electronic components to go wrong and to try and find a problem you need a diagnostic do-hickey at a cost of thousands of dollars or own a dealership. I would love an E39 M5, but by the time I could afford one it would need a lot of work and the cost of replacing somethings would be prohibitive. The new cars are made virtually for one owners and then recycle.
The E30 is probably the best to play with, then the E34 IMHO.