I use SB400 when I can and the SB600 when it's required. I can get 2 SB600 for the price of one SB900 which I would prefer.
The SB400 recycles faster than any of them and is great for daylight photography...fill. I angles only up, but most of the time that's plenty for me.
It really depends on what you want to do with it. I like to lug as little stuff around as I can, so the small form factor of the SB400 is perfect for me. But if you must have more power, larger ones are more suitable.
Good luck with your choice!