You can't just say what's the best rom because every rom it's different just like the users and how they use their phone.But if you wanna know some quick facts here we go:
Stock GB has the best battery because it lacks feature and with a good kernel it can be pretty fast.
Now talking about custom roms :
, in my opinion has better battery than JB or KK. Free ram is pretty high,second place,first is KK.More features than GB not so many as JB or KK
: not much of a big fan but i can say that 4.1.2 was by far the best from all the versions.The battery and resources consumption was fair and has more features than the precedents.
is my favourite.The battery consumption it's ok and it has more free ram than any other android versions and the features are just great.
As for kernel i'm a big fan of Stratosk's Semaphore kernel.No matter what rom i use the kernel is the same.
If you wanna try custom KitKat rom you can try these ones(these are the only ones that i have tested like 2 to 4 weaks each)
SlimKat - Very very smooth just like all their custom roms.Nice features like TRDS, SlimPie and others. Ram usage is good but the battery is draining a little fast for me but this is just my case and maybe others have different experience.
Mackay's Kitkat - This is also a very nice kitkat rom.it's smooth it has good features but just like the one above, i don't like the battery and after some time it became a little laggy.
C-Rom - This is the last one which i'm using right now.I love this one.Ram usage is good , i like the battery( i can get over 19 hours) ,it has great features.Never felt my i9000 so smooth.
My test where done with 3g/wifi on all the time,more than 3 hours screen time,music for 2-3 hours,facebook,whatsapp and others(daily).Some of the apps are greenified and some frozen.
Hope this helps you !