Your main. Seriously.
Try it with people of different speeds, weight, and traction (especially traction). You can find stats in the stat list. Compare different stats in order to better understand the engine.
Also, since a wavedash is just the excess of a dodge into the ground the furthest wavedash is at about 20 degrees, more to the side than down. Going further down inangle will make for a shorter wavedash.
Try it with Luigi (longest), Zelda (one of the shortest ones), Bowser (Slow timign), Fox (fast timing), Mario (pretty average), Ice Climbers (so very fun to do for some reason).
Easiest is probably the middle length wavedashes: Samus, Marth, Mario, Doc, (everyone claims Luigi to be the easiest).
Hard ones: Fox (some claim it to be the easiest because it's just fast, no real rhythm), Ganon, Zelda, Link, Bowser, Peach . They all seem to have slightly different timing from most.