The Neoya one seems to be the most popular as long as your screen can do the upscaling, from what I've read it works with all the types of signals that the Wii can output (480i, 576i, 480p, even 240i for a few VC games apparently?). Ordered mine a few days ago. I'm personally inclined not to trust any of the products with a built-in upscaler just because of the chance the quality will suffer slightly as a result, which is why I went with the Neoya one, because it doesn't seem to mess with the video signal beyond converting it to HDMI.
Interesting discussion here: http://www.neogaf.com/forum/showthread.php?t=444456