Probably the smoothest/best ROM.
Helly Bean 4.1.2 with a Marmite kernel is what I have been running for the past 2 months.
Stock 4.1.2 lags. So in February, I went to:
-->4.2.2 has lag and a lot of stuff I do not like (lock screen swipe camera, slow lock/unlock, alarm, camera UI).
So I went to 4.1.2 (Marmite kernel):
--> CM 10 is slow, and lags
--> Slim Bean 4.1.2 is great! But the battery is horrendous
--> Helly Bean 4.1.2, just about perfect. Good battery life (although I don't like it, it's probably the best the Nexus S can do). Great performance and features.
One app I think aids in my current performance is Greenify
. It has great reviews, and is probably also contributing to my phone's experience.
If you are trying to find the smoothest/fast/good battery life/reliable/best ROM combo, try out: Helly Bean 4.1.2 + Marmite kernel + Greenify
Nexus 4 - 16GB Black, Purity 4.4.2, Franco Kernel