Of the three you listed there, the Toyota system is probably the most advanced simply because it was the last one designed. VTEC was so widely "popular" because it was the first out, and came in some pretty potent sport compacts. You may think that VTEC is bar none the most popular, but that is because VTEC is the one people brag about most. All of Lexus' line has featured VVt-i for a few years now, but they don't have the huge stickers on the side because 45-something father's of 3 don't want stickers on their ES300. The GT-S on the other hand screams of its VVt-i system. Not many people know this, but not many companies have actual variable valve timing, the majority, including BMW's VANOS system is variable cam timing. There is a slight difference. The VTC part of i-VTEC is simliar to VANOS. To my knowledge, there is only 3 (4 if you count i-VTEC) kinds of variable valve timing on the market right now. VTEC, VVti-, and MIVEC. Of those 3 VVt-i is probably the best because the variation is done in the head, not in the cam as is the case with VTEC and MIVEC reducding paracidic loss to wight in the valve train. BMW is comming out with a system to match to its double VANOS system which consists of rocker arms situatuated on a movable fulcurum, allowing for infinite VTEC (not two stage anymore). That system will be the best once it comes out. The way I remember I tell the difference between variable cam timing and varible valve timing, is that vvt changes the actual lift and duration of the valves (as can be seen by the fatter lobes on a vtec cam). vct only changes the advance or retard of the cam relative to the speed of the engine, in essence only allowing a longer duration. A vvt system is a little more complicated, but allows for more power potential, as we all know because VTEC kicks ass!

PS I don't know much about MIVEC. I don't think it was ever offered in USDM. Does anyone know more about it?