It's easier to just point you in the direction of MusicCity's article on airbrushes. That will teach you everything you wanted to know about external vs. internal mix, gravity vs. siphon feed, single vs. dual action as well as the varied means to supply air.
Which is the best is completely up to the user. Airbrushes are tools, and the best hammer in the hands of a newbie is going to bend nails. It takes practice, practice and more practice to become proficient in using this tool. It's also going to be a major investment for anyone to get into a complete airbrush system.
One thing I've learned since getting into airbrushes, is that they are both easy and frustrating to use. Spraying your first flood coat of paint will make you wonder why you waited so long, and your first crack at camo will cause you to think you've made a terrible mistake.
If you're anything like me, you won't settle for just one, here's my stable:
Badger 100LG: Top mounted gravity fed dual action internal mix with medium needle. I actually have another 100LG with the fine needle. The color cup, while called "Large" isn't and requires refilling for large painted areas. This airbrush gets my "Cold Dead Hands" award.
Badger 150: Bottom mounted siphon fed dual action internal mix with medium needle. This shares the same parts as the 100LG (except for the actual airbrush body) and was my first airbrush. Came with both a 1/4 oz. color cup, 1 and 2 1/2 oz. bottles. It is a good brush, but I find the 100LG a little easier to clean, so the 150 stays in the toolbox. This brush could be pried from my fingers, but I'll put up one hell of a fight.
Badger 200-20: Bottom mounted siphon fed single action internal mix with medium needle. This shares the same head assembly as the 100LG and 150, but the needle is significently longer. This is considered a "detail" brush, and since you dial in the needle position I can spray a finer line with this brush than the 100 or 150. I find it more of a pain to adjust the needle and it takes a bit of trial and error to match previously sprayed lines, but it's a darn nice brush and also gets the "Cold Dead Hands" Award.
Clone of a Badger 350: Bottom mounted sphon fed single action external mix. I bought this for $10 at a discount parts store. It does a surprisingly good job of flood coats, and I keep it around for spraying Future as clear coats.
If I had to do it all over again, I would go buy the Badger Renegade Velocity. It's the newest version of the 100LG, though all new parts. It's very similar to the Iwata HP-CS and the Paasche Talon.