It took me a while to catch on..
ebike controllers come with 2 flavours of connectors, the 'Higo' or more standard wire connectors, like in the pic
@saneagle posted above or the 'waterproof' Julet plug and socket connectors. and its a real pita trying to wire a Higo terminated sensor into a Julet socket and visa versa.
Then since the headset and controller 'talk' to each other they must be compatible so its always wise to buy the headset and controller together..
KT controllers are popular as they provide access to a number of config parameters generally locked out by eu legal ebikes and some equipment distributed locally.
and they come with a lot of headset options some very simple, but if you want to configure the controller go for a 3 button lcd headset
Thirdly the signal generated to drive tthelarhe motor can be either a square wave or a sine wave..
All i know about this is sine wave controllers are preferred as they provide a smoother drive with geared motors..
So once thats all sorted you can select the controller with the correct voltage and amperage for your motor and needs..
hope thats helpful..
EDIT** If you look at the price of brake and pas sensors etc and think its worth going julet and replacing your current sensors etc you will also need a 4 into 1 or 5 into 1 'bus' cable
and a motor extension wire, a real pita to discover after waiting 3 weeks for a delivery from china