I will assume you need it for circuit board work, a temperature controlled unit would be best, but i use either a 12watt and a 18 watt for small circuit work, and I have a 30 watt iron and 100watt soldering gun for cable work and fitting solderable plugs(PL259 etc.)
It all depends on what you intend using it for, hope this helps, Karl.
PS, whatever iron you use, keep the tip clean, and it will last a lot longer.