Mango
I have a Kala mango uke, thought not solid mango, it does sound VERY nice acoustically, I'm not crazy about the shadow pickup in it, but that's something I can deal with as I haven't been using it electrically very often as of yet. For me, it's similar to what others have said, very similar to mahogany but slightly different, but very beautiful. If I had a real choice at the time, I probably would have gone with Koa, but being left-handed and finding the lefty Kala in mango was a sign, and i'm glad I gave it a shot, rather than buying something more expensive and converting it to lefty...
Paul