First move choices  

Here are the possible opening moves on a 9x9 board, in roughly descending order of popularity from a set of pro games (largely taken from Gobase).

[55]

5-5 opening  

The 5-5 point seems to be the most popular opening.

Monteo This is the best opening move, particularly for the games with high komi (e.g., 7.5 points). Go Seigen, too, played this move and beat Miyamoto Naoki by 4 points.

[34]

3-4 Opening  

There are many choices after 3-4, but of these, h-k have appeared only once in the set.

bugcat: For what it's worth, a dan player once told me that they had never lost on 9x9 against an opponent who opened with 3-4.

[44]

4-4 opening  

Choice a appears about twice as often as any other choice.

[53]

5-3 opening  

Choice a is about twice as popular as anything else, and f appears only once.

[54]

5-4 opening  

Choice a is more popular than the others.

[33]

3-3 opening  

No doubt other choices are possible as well.