I've used 22, 25, 28, 30, 32, and 36 point buy.
22 Point buy is what Living Greyhawk uses for cohorts. It's possible to make an effective character but requires using at least one dump stat. I wouldn't recommend it for a campaign.
25 Point buy is used in the Legacy of the Green Regent campaign. It's rather limiting in terms of what can be done and certain characters (monks, paladins, etc) will tend to look rather cookie cutter since they don't have a lot of flexibility.
28 Point buy is what I've used in Living Greyhawk and a couple of home campaigns. You can do a fair amount with 28 points including multiclassed characters.
30 points was what my group used for the Return to the Temple of Elemental Evil. My archer cleric was more successful with 30 points than with 28 but, on the whole it is similar.
32 points is what the Living Arcanis campaign uses. It allows more flexibility than 28 points but, IME, it seems that more players have managed to take that flexibility (and a number of high-powered Arcanis races) and create ineffective characters with them. I see a lot more uselessness in Arcanis than in Greyhawk.
36 points is what I mistakenly used for first 3.x campaign I ran (I thought it would give numbers similar to what we generally got with (4d6-L)x3. It's also what the home campaigns I'm playing in ATM use. In my experience, it's roughly equivalent to a +1 ECL race and, like ECL races, tends to result in a more deadly game since greater challenges are necessary to compete with the more powerful (offensively) characters but the characters' defense isn't actually equal to the challenge so PCs are more likely to be killed by a bad roll. (Incidentally, my experience with that campaign and early Living Greyhawk also demonstrated that high stat points don't actually make up for low magic. Since magic enhances characters' abilities in a number of ways that stats don't, the more stat-dependent campaign quite quickly ends up favoring characters who depend less on items--barbarians who figure they'll get hit anyway over fighters who try to go the defensive sword and shield route (you can't maintain this without getting magic armor and shield, etc), multiclass clerics and wizards who can use spells to buff themselves over fighters, etc).
If I had to choose a single point buy, it would probably be 28. The game seems to work best there though 32 points works as well.