OK, here are my smallest part counts.
Skipping 1: 0 pieces and Scott's perfecting my 1 piece #2.
3 "Mind The Gap": 1 piece Leaning tower of Pieces
4 "Junk Yard": 4 pieces AWD... FTMFW
5 "The Wall: 2 pieces Rolling Tower of Pieces.
6 "On a Roll: 1 piece Lollipop
7 "Full Up": 3 pieces Extension Ladder
8 "Higher": 2 pieces Levering Tower of Pieces (This one is Similar to Scott's but backward and box is held above the line)
9 "Around The Bend": 0 pieces Boring but obvious once you figure out what speed makes it bounce back and not get stuck...
10 "Up The Hump": 3 pieces Roscoe P. Coletrain landing.(Again, similar to Scott's)
11 "Mission To Mars" 1 piece Circus Act
12 "The Stairs": 4 pieces AWD Rag Top FTMFW again...
13 "Big Ball": 3 pieces: (I got rid of the top roller!)
14 "4 Balls": 7 pieces: Spinning Wheel with passing.
15 "Down Under": 6 pieces: Swinger
16 "Awash": 7 pieces This level is just a mess... It makes it eventually.
17 "Handling": 4 pieces: OK, Scott, I even set the piece down in the box with less lego.
I still like my
5 piecer with suspension better.
19 "Back and Forth": 6 pieces: Duke Boys & General Lee
It is going to take a wile to bring down the part count for 18 & 20...
18 "Tube": 17 pieces (I think) : Elevator
20 "": 21 pieces I think 
The best of all my other levels. Extension ladder, AWD, Swinger, etc.