I like the Badger 100/150/200 series of airbrushes. They're a bit thicker than a pencil, about the size of the fine line Sharpies. I'm perhaps biased as I have one of each! The nice thing about that is that many of the parts are the same, so I don't have to keep a stock pile of parts.
Which one is dependant on what you expect to do with the airbrush.
The Badger 200-20 is a great airbrush for beginners, or even "experts". Single action siphon fed with the PPS paint measuring system that allows you to "dial" in the thickness of line for consistancy. It can be found at Dixie Art for $49.00 including shipping as long as you exceed $60 in total purchases. Can use bottles or color cups.
The Badger 150 is the dual action version minus the PPS system. Great for spraying large solids, sells for around $63. Can use bottles or color cups.
The Badger 100G or LG is the dual action gravity fed version, minus the PPS system. These are often the brush of choice for many modelers. Gravity fed brushes are arguably better for fine lines, as they require slightly less air pressure to spray the line. Siphon fed brushes require a higher PSI to pickup the paint from the bottle or color cup. Goes for $69. The difference between the G or LG is the size of the fixed color cup.
If I had to choose only one airbrush, I would go for the 100LG, though the 200-20 comes really close. The 150 is an excellent brush, but it doesn't do anything the other can't do, and can't do some of the stuff they CAN do.
My recommendation to people wanting to buy their first airbrush is the 200-20 for it's simplicity and ease of use.
I've heard nothing but good things about Iwatas, but I've also heard that they tend to be a bit finicky, where the Badgers are perhaps a bit more forgiving. They also tend to be more expensive, though the Revolution series is pretty much the same as the 100/150s. Paasche are another very good manufacturer, but they tend to be a bit chunky in size. I found them uncomfortable in comparision with the Badgers.
Grab a Silentaire Scorpion I for $149 at Dixie Art, a few spare needles, head washers and o-rings and a bottle of lube, and you should be able to get the whole package for $250 with enough left over for a kit and some paints!