Which shaft diameter you think is the best overall for playing 9 ball?
It depends on the player.
Large hands, large forehead with big eybrows, likes to drive pickup trucks, 13mm
Smaller hands, likes to drive Prius and wear cologne, 12.5mm
Funny question if you've been playing for more than a few weeks and have been around any type of real players. For some reason there is another guy with few posts asking some very noobish shaft questions at the same time.
Here are some more fun topics to find:
What type of cue should I get for a 7' vs a 9' table
What tip is best
What shaft is best
Should I change shafts when I use draw vs follow vs center
There are few games were you would use a different type of shaft. 14.1 requires a lot less ball movement than almost any other pool game, you may want a stiffer shaft that has less "action" in it. Snooker you want a small tip due to size of the balls. 3C you can also use a specialized cue or shaft for.
Diameter of tip for 9 ball? Does not matter one bit.
The general rule is: Smaller Tip = easier to get spin but needs more accuracy, Larger Tip = less spin but has a larger margin of error on where you can hit the cueball. Same thing for tip radius, dime radius = easier spin but needs more precision, nickle radius less spin with same effort but you can hit a bit more sloppily and not miscue or send the cueball in some odd direction.