First find a company that you really like. Regardless of what you get, you'll know that the quality is good.
Secondly, decide what you'll use the cajon for. This will help narrow down which one is best to get FIRST. You can always buy more! LOL
I had
Paul Namkung custom build a cajon for me. He only makes about 50 a year and spends most of his time building taiko drums. Here's a photo of the drum he made for me.
The drum has two playing surfaces, one on each side of the ported side. The panels are solid, not loose for the
crack sound, and no guitar string inside, which is used to be a
buzz sound.
I then bought the cajongas, which Gaddabout mentioned, from Schlagwerk. These are small drums that are played like congas, but sound more like a cajon ... thus the name
cajongas.
They have vicro-strips on them so you can connect together to play as one instrument if you like. They sound very, very cool!
Schlagwerk makes a neat drum called yambu', which comes in a conga and tumba size. The drums are played like congas, but have the guitar string-wire inside which gives it a cool
buzz effect.
What's cool about ALL of Schlagwerk's products is that the items with wires and be turned on and off. It's not easy like a snare strainer, but it works.
The Peruvian style of cajons have the loose panel so you get the
crack or
crash sound when you play the edges. These are very popular for
flamenco styles of music.
Flamenco is the dance, Flamingo is the bird. 
Personally, I would get a drum without the loose panel ... as my first drum. The characteristic sound may be limiting IF you plan to use it for drum/perc loops, etc. If you get a drum that you can turn the buzz on and off, as I mentioned, then that would be a good first drum.
I'm now needing a drum WITH the loose panel or wires. It may end up being one of those yambu' drums.
Oh, one more FYI ... I personally know several
top name percussionists who are LP endorsers ... and the swear by the Schlagwerk products. I have to agree with them. The Latin Percussion cajons sound almost as bad as their Mark Tree(s) ... and that is NOT good.