Oaktons.... use Oaktons unless you want to fork out the big bucks.

Hanna makes nice products when you buy their more expensive pieces ($2-300 per individual tester). Their less expensive testers are fragile and need constant recalibration. Don't buy their combination units either because they break more frequently and are typically less accurate than separate meters. Hanna also is a pain in the ass when it comes to warrantying-out their items.

Oaktons are cheap (considering what you get) and they last forever. I use the ultrabasics and when I check the calibration on them every month, my pH meter is seldom off by more than .1 and the TDS changes by only about 20 ppm.

Whatever you decide on, buy a tester with a replaceable electrode... it will save you money down the road.