I use a Kinesis number pad with Browns I found on ebay for $30 new in box. I don't know that I would call it the best, but it uses plate mounted switches, has a USB hub and sends number pad scancodes. Stock caps are pad printed ABS, but that's easily fixed. Of the commercially available options, it seems really good for the price.
The only real flaw I could say about it, it maintains its own numlock state so numlock itself doesn't send a scancode. I don't find it a problem, but I could see that being a deal breaker for some.