From my tests (which were done in less time than it took to write this post, with the method described below), both simple running and rolling in ball form are equally the fastest ways of travelling. Jumping (technically, it is landing, not jumping) adds a small slowdown needed to regain the velocity, so it's better to avoid it. Falling (that is, moving down and in the horizontal direction without jumping prior to it) also reduces your travelling speed (note that the aforementioned slowdown occurs in this way, too!), so it's better to do a small jump everytime you need to move to another platform than just fall on it from above.
Anyway, there's one near-100% accurate way to determine it for sure, and it is suitable for every game out there: 1) load it up in the emulator; 2) choose a place suitable for measuring distance, define points A and B (the bigger the distance, the better); 3) turn on frame advance and start recording; 4) move from the point A to the point B in a desired method of travelling; 5) upon reaching point B, note the number of frames it took you to travel there; 6) stop recording, return to point A; 7) repeat steps 3-6 as much as you need.