Personally I've always wanted a M30 3.5 in an e30. Light shell and low down torquey strong engine.

But its your car so depends on what's available out there for the right money and how handy you are with spanners. If you want to stay 4 cylinder how about 318is engine from an e36 with the later m44 engine.
6 cylinder wise probably m52b25 or m52b28 - you can buy e46/e39 cars cheap enough with these engines
And if you want to go balls out then s50/s54/s62 etc but big money