the -400 for me, standard sized and very popular.  The -900 is too big to be a 737 for me, I prefer the smaller variants

The 200's are the coolest!

The 100's are interesting... here is their story:

Boeing decided to steal the exact engine setup as the 727. The engine (JT8D), AND the engine mount and pylon! They just turned it 90 degrees, stabbed it on the 737, and went!
Problem: The reverse thrust system was also the same...... BAD! It would cause a high pressure under the wing, reducing brake effectiveness to the point where it would take too long to stop.
Solution: Boeing designed the nacelle longer for the 200's so it would be behind the wing, and exen angles them for maximum effectiveness. I don;t know if it is still true, but for many years, the 737-200 had the most effective reverse thrust stystem on the planet due to all the time Boeing spent on fixing it.

Besides, how many other 737's (or any other turbo-fans) can land on short gravel strips APPROVED? (See: Alaska Airlines) The 727 does OK (See: Northen Air Cargo), but not as well, because of the superior reverse thrust of the 737-200.

I recite myself from a post I made in januari, regarding the 737-200


"....Changes included new high-lift devices and braking improvements and low-pressure tyres, as well as new deflectors fitted to protect the lower fuselage and engine intakes from scuffed-up stones and debris on rough runways. FAA certification for gravel runway operations was forthcoming in February 1969"

It's no secret that Alaska, Wien Consolidated, Nordair etc used the 737 on unpaved runways, why is that so hard to believe? geez....  ???

Close-up of the deflectors
http://www.airliners.net/open.file/0237317/L/ ;