They all work. The ones with the remote shunt are not waterproof and designed for panel mount, so you have to put them in a box of some sort.
The GT one is the easiest to waterproof, but like the other non-remote shunt ones, if you want it on the handlebars, you need to modify them for a remote shunt, which means unsoldering the shunt and making your own shunt out of a short length of 14g wire (your battery cable).If you don't already have a means of measuring current, it's best to buy two so that you can use one to calibrate the other in order to get the length of wire in the shunt correct.
If you're happy with it adjacent to the battery and not waterproof, any of them are OK and easy to fit.