What are the pros and cons of different step up transformers?

I am doing alot of my schools science projects on high voltage electricity, so I am dealing with step up transformers.





What should I use as THE best step-up transformer based on:





Winding resistance, eddy currents, hysteresis, magnetostriction, heat by copper and iron loss, stray and mechanical loss.





Toroidal, air core, EI or powdered iron cores?





Reply's will be much appreciated.|||Powdered iron cores are the best