depends, if you wanna use a older firmware, then i'd say ITA 4.50, as it is quite stable though it gets more errors than newer firmwares and you will not be able to use disc movies. Though release mode works better on the old ITA then it does on any newer firmware, but i use cobra so that might not be a completely true statement.
Now for newer firmwares it depends if you wanna have webman or not in my opinion, my personal experience 4.75 ITA is a good firmware but the MAMBA stuffs up after awhile sometimes as its not perfect so in this case rebug is the go, that and rebug has some useful options such as cfw tools and access to all rebug toolbox options, atm best firmware in my opinion is 4.80 Rebug i've not had a single issue with this firmware yet so yeah it would be the way to go, the biggest difference that can be seen is the icons are differnt color, coldboot is differnt and install packages in differnt but you can change all those aspects yourself so doesn't matter anyways