This is a very subjective topic, but for me, koa tops sound their best with bright fluoros installed. Worth Clear, Oasis Bright, Martin M600 etc. In my mind, something about the natural "crystalline" tone of koa is enhanced by the added sparkle of a nice bright fluoro string.
I have enjoyed playing primarily on cedar and redwood topped tenors for the last couple of years because I like the crisp, guitar-like sound and extended sustain that they produce. Recently though, I have been becoming more and more intrigued by an all-koa concert scale KoAloha, which I have recently strung up low G, and which has quite a complex and interesting tone with the right strings. I have been so impressed that I find myself wondering if a tenor-size koa uke might be in my near future.