"Best" is a difficult term to define and endlessly arguable. Suggest you focus on what self defense ammunition gets consistently good results in objective ballistic tests, has proven effective on the street, and generally gets good word of mouth.

I'd discard the search for "cheapest", although there's nothing wrong with finding a good deal on quality, established ammo.

Another factor to consider is what weapon you're running it through; that affects performance and so choice.

There are many good options. I suggest Speer Gold Dot. Unless you're using a short barrel (under four inches), suggest you get yourself a supply of their 9mm in 115 grain, 124 grain, 124 grain +P, and 147 grain, and shoot a couple magazines of each to determine how your individual weapon in your individual hand responds to each one (accuracy, reliability, felt recoil).

Whichever one you like the most, make that your self-defense ammo of choice.

Ammunition To Go has the law enforcement 50 round packs of these at excellent prices.

As a back up, although I've never used it I've researched CorBon's DPX line of self defense rounds and am convinced of their efficacy.