what is the suitable kernel with rootbox 4.2 ??
I'm a fanatic ex RootBox user. To me, this question rather depends on personal preferences. There are no such thing as this kernel is better than that kernel because in my opinion, all items on XDA are treated the same respect. If you have to ask me, I'd stick to the stock kernel that came with RootBox itself as I think the developers developed their ROM to proper blend with the kernel they provided, just make sure they are compatible with each other.
Ever since bajee has left the development, I switched to NeatROM JB 4.1.2 Aroma (and never regreted once) since they are more active supports, updates, and mods for this ROM. It's very sad though RootBox has so much potentials to stay on my top ROM list.
You've gotta try to understand and satisfied with the kernel you choose for RootBox and the answer will come from yourself. People can make reviews, but that's all depends on how they use their phone. Same questions like battery life, etc. it's all depends. To me, the best combo is Apollo regular + NeatROM JB. Best battery life ever, you will be surprised looking at the battery graph.
Hope that helps.