There are lots of EC pens out there but the undeniable king both in consistency and value for money is the Bluelab meter. 
 
It provides quick and accurate readings and is easy to calibrate. We have used many EC pens in the last 2 years of growing with TowerGarden and this... just works. Alongside this, if you want to be super on the ball, it also takes the water temperature. Perfect when you need to get your jacuzzi just right! But it can be useful to understand water temperature if you have any issues, albeit it's very rarely needed. 
 
It doesn't take ages to provide a reading, it doesnt need constant recallibration, it's just brilliant.