I might be missing the point, but
1) Heating element in the flow
2) Pipe Heater wrapped around the outside of the pipe
3) Magnetic Pipe wrapped with an induction coil
4) Pipe wrapped with a steam jacket (not electric ignore?).
Of these, the first 3 are the simplest. The 1st one is the cheapest, the 2nd and 3rd approaches are more expensive, but the pipe is heated externally, so might have the advantage of being easier to clean.
I have used the 2nd approach, and am working on prototyping the induction + magnetic pipe approach.