318i - From '84-'85, the 318i came with a 1.8L four cylinder engine that made about 100hp. BMW then dropped it from the lineup, but resurrected it in 1990 as the 318is, but with a new DOHC engine that made the car much more spirited. It can be easily argued that the 318is is perhaps the best handling E30 (besides the M3), because of its low weight, and better weight distribution.
325e - The 325e was produced from '84 to '88. It had a 2.7L inline six cylinder engine that was tuned for economy and plenty of low-end torque. It had 121 hp and 170 lb-ft torque. This made it a real joy to drive around town, but out at the track it would fall to the M3 (of course), the 325i, and the later 318i's. The engine, although of larger displacement than the 325i's, incorporated such things as a four bearing cam (as opposed to seven in the 325i), and looser valve springs to increase efficiency. The redline on this engine was very low, only 4800, I believe. These are popular engines to modify, because they are cheap and easy to find, and the larger displacement is a good start for a hot-rodded engine. With some cylinder boring, custom pistons, and a donor head from a 325i, it is possible the build a real screamer 2.8/2.9 L "i" engine.
325i - The 325i was introduced in '87 and ran until the very end of the E30 in '91. The engine was only 2.5L but it was of much higher tune, revving freely up to 6500 rpm, and producing 170 hp and 164 lb-ft torque. Despite the excellence of the 325i, I don't have a lot to elaborate on, other than to say it was a very quick, beautiful car, and that I was lucky to stumble upon such a well kept example (at a very good price, I might add).
M3 - The pinnacle of E30-dom. Although it was an E30, the M3 was a very different monster. The engine was only a 2.3L DOHC four cylinder to keep weight and weight bias in acceptable ranges (later available in 2.5L in the EVO models), but was an engineering marvel that revved easily to at least 7000, and pumped out 200hp. The engine itself was a hand built one-off race engine that BMW produced to satisfy the displacement restrictions of the racing class the M3 was meant to compete in. The entire suspension was given a thorough once-over as well, with stiffer springs and shocks, larger wheels, slightly different suspension geometry, and relocated mounting points for some parts.