"Best" ROM.There is no such thing as a best ROM. The question itself is ambiguous. "Best" is obviously a subjective term.What I want from a ROM may well differ from what you want from a ROM, ergo - what is best for me could be worst for you.If you are asking what the most popular ROMs are, or which ROMs people are using, you can see which threads stay around on the first few pages (and have the most posts) in the Android Development or Original Android Development forums. You can also see what other people are running by reading the What are you running on your Nexus 5 thread.If you are asking which is the most stable, being a Nexus device - they're all pretty stable.If you are asking which is best on Battery, ROMs only affect battery if they have a feature that is badly coded. You will likely be able to read about this in the ROM threads. ROMs do not impact battery life. The only impact to battery life are your apps, your settings, how you use the phone and mostly, environmental issues such as Phone Signal.For tips about improving battery life, please read [Battery Life Help] Troubleshoot battery issues here! "Best" KernelAlso, please note that as above, "best" is still subjective. What we all want from a kernel is different. Again, many people have the misconception that Kernels affect battery life. Let's get this cleared up. Although Kernel devs will build in optimisations and efficiencies that will improve battery life, these are very, VERY tiny...and if 1 kernel has these optimisations, they likely all have.People will often say "Kernel x is better than kernel y for battery life". This is actually wrong. Kernels respond to user settings. Setting up the governor to favour either battery life or performance is simple enough to do, you just have to do some learning. The reason people think Kernel x is better than y is because developers set their kernels up with their preferred governor settings. This is what we refer to as out-of-the-box settings. The out-of-the-box settings for kernel x may well produce better battery results than the out-of-the-box settings for kernel y, which favour performance. The fact is, you as the user have the ability to tune kernel x or y to perform the same, be that battery or performance - so start learning how to do this yourselves - that way, you can choose the kernel based on the FEATURES you want, and not the fictional performance benefits of one kernel over another.Hope this helps