I have quite a few contact mics ranging from cheap DIY ones, to my workhorse K&Ks to a fancy-pants Schertler.
In general I use the K&Ks as they are cheap, compact, and relatively sturdy. I have several ‘hot spots’ and a ‘double hot spot’. These are quite quiet (very little noise/hum) and have good frequency response (bass is still there, and they are really bright).
The Schertler is deathly quiet when it comes to noise/hiss. Like night and day difference from any other contact mic I’ve used. It sounds great as well, though significantly less bright than the K&Ks (surprisingly). For this reason I would rarely use it completely on its own, since it lacks that sparkle.
For DIY things I’ve made mics out of all different sizes/types of ceramic piezo discs, and even made several using piezo film, which is supposed to sound fantastic in certain contexts (double bass bridges etc…). Unless you shield these really well the noise is often prohibitive.
So given how cheap K&Ks are, I’d go that route, if you’re going for a Daxophone type thing.
In general they all sound a lot better with a buffer in-line, at minimum. I’ve got a few of these I’ve built:
I also have a stripboard layout that I had someone work out for a Barcus Berry 3000A preamp/buffer that fits in an altoids tin (along with a 9V battery). It sounds fantastic and has bass/treble/volume controls, so super useful/flexible. I’d post it here but I got the schematic for the BB preamp off some random webpage, so I’m unsure of the copyright situation on it, but if anyone PMs I’d gladly share the stripboard layout of the circuit.
I also can’t recommend this book highly enough:
Lots of great info about contact mics in there, but also covers all kinds of pickups and approaches to amplification.