1) E46 M3
This is an iconic car that is recognized internationally as a performance benchmark among its peers. A nimble, driver focused car with a gem of an engine. A low mileage good condition example can be had for less than $25,000 (CAD). There are a lot of good upgrade options by using BMW competition pack and CSL parts.
2) E30 M3
Pure iconic classic - need I say more. I fear once you have one you may never sell it.
3) E39 M5
Arguably one of the best sedans BMW has ever made. Smaller than an F30 3 series it is very nimble with tons of feedback to the driver, with a design that somehow doesn't feel dated and a robust interior that in most cases has held up very well.
4) E9x 335 & 335is
When this car was released in 2007 it completely revolutionized the German compact car class. No other car on the market had the same power, smoothness, and dynamics of a 335. Known for their ease of tuning, turn one into a dedicated track car, street sleeper or just keep it as an awesome daily driver.
5) E9x M3
A future classic, and the last of the naturally aspirated M3 cars. It's modern enough but can still be had with a 6-speed, no idrive, and a carbon fiber roof.
6) X5 4.8is
Truly a monster of a vehicle, and really represents a good upgrade over the 4.6is - doing 0-60 in 6.0s flat, and with a stock exhaust that sounds like thunder, I could not imagine a better naturally aspirated v8 suv. Down side - these SUVs tend to need a suspension refresh especially with higher mileage vehicles (I know through personal experience with my friends' X5 4.6is)
With the engine from the E46 M3 and the most unique body in recent BMW history, what is there not to love (even though some people call it the clown shoe - at least its a fast shoe).
8) E82 135 & 1M
9) E39 540i m-sport (sedan & wagon)
To me the E39 represents the pinnacle of BMW sedan perfection, 4 doors, the same size as a modern 3 series, excellent driving dynamics and a sweet 4.4 which in later revisions was good for 291 hp and 340 ft/tq. The m-sport package really makes the car look sharp and purposeful and is no doubt the one to have. Of special interest is the 540i m-sport wagon, it was the sportiest and fastest wagon that BMW made in the E39 generation and is no doubt a rare and special car. This would undoubtedly be my wagon of choice for its power plant, looks and dynamics.
10) Alpina B7
If I was in Europe I would say the Alpina B3 not the B7 but unfortunately the only Alpina brought over to this side of the pond was the B7. Every once and awhile even the most hardcore track fan would love to cruise the streets in a big powerful sleeper sedan. Nothing really beats the refinement, design and details of an Alpina. I think the blend of body, styling, and performance is the perfect upgrade over a 750i. It also doesn't hurt that it hits 60 faster then most sports cars.