The LR Baggs 5-0 is probably the most natural sounding one I've come across. 300 hours of play on a small coin type battery and volume control included.
I had the Pono passive pickup in my last uke, and the K&K Aloha Twin Spot in this one.
I honestly can't tell the difference through my crappy "rock guitar" amp.
I found this video extremely useful, and it's what I used to pick the Twin Spot this time around...