The Squid
Spaced-out CephaloMOD
Depends on your price range really. I use the Hanna combo, it measure everything. Ph, temp, ppm and it's waterproof and really nice, top of the line pretty much, although I am sure one could spend more. It's $129 US. Either way you'll need calibration solution and storage solution for your digital meter. Hanna makes one that just measure ph which is fine. That one is $70-80 US.