I was planning out a new build which in which I'm planning to use the L9i as the CPU cooler.
What's the best it can handle? (I'm not really planning on overclocking, but it would be nice if there will be some headroom)
Short answer is it depends. If you are asking what CPU can the L9i can handle, check out this link from Noctua. Technically it can "handle" the 9900K (with a severe power limit). It would also depend on your case, air flow, and willingness to adjust power limit / boost / AVX / undervolting / etc.