I don't know if there is a "best" format, it's probably a matter of opinion. but I personaly prefer tiff (I always render in tiff). Because it has lossless compression (so in other words I get perfect quality like with say a bmp, but I don't get a huge file, tipicaly they end up smaller than a psd (photoshop file)); also because it's supported by every program that I've seen; and it even suports layers and masks (alpha channel).

After I have the tiff then I use photoshop or fcheck to change it to whatever I need (jpg if I want to post it on the web, psd if I did something to it in photoshop, bmp if I want to use it again in maya as a texture or something, etc...)