M3 e36 I reckon.
I'm pretty sure the value of a decent one is going to up pretty soon. E30 style.
The e46 is better in every way and can be had for <10k.
I can't see why anyone would want to spend the same amount on an e36 over an e46.
Having said that e30's fetch 20/30k+.
Here's my input:
Anything that has the following badges will always become desirable:
M (BMW) - obvious reasons - badge/rwd/speed etc
RS (Audi) - not sure about classic desirable but desirable for it's nice soft-feel plastics or perhaps its blistering pace.

Specifically the RS6 - could imagine the older types wanting them.
RS (Ford) - as mentioned a lot previously "the working man's hero". Always collectible.
Type R (Honda) - not hugely desirable as marques go, but the ek9 warrants big money (a few k more for a '98 ek9 than a '02 ep3 for example). Perhaps in 10 years the ep3 will start to be worth something (the last of the true Type R's?).
GTR R32, R33 and R34 (Nissan) - When the PS2 generation can afford them they will become very sought after.
Impreza/Evo (Subaru/Mitsubishi) - 22b already is and the RB320 will be and perhaps the RB5; also some others I can't think of (GB270 or something and the WR1 - the WR1 simply to the sheer performance it has). The Tommi Makkinen is close to becoming classic now (they fetch 10k+) and give it another 5-10 years they will be like what Lancia Delta Integrales are now (20k+).
Coupe (Fiat) - especially the 2.0 N/A 16v as I believe these are the rarest and considered better by enthusiasts than the 'gary-boy' turbo version.
Someone mentioned the ST220 earlier, I personally can't see that getting too collectible - much like the Sierra Sapphires are not as collectable as their sierra counterparts.