Mnikolic

From the storyline's side:
1. In "Day Of The Dumpster", Zack, Kimberly, Trini, Billy and Jason get chosen by Zordon and become the world's very first modern-day Power Rangers.
2. In "Wild West Rangers, Part II", Kimberly is trapped back in time and recruits 4 teens. Those teens (minus Kimberly) become world's ancient Power Rangers.
3. In Wild Force, in distant past, Merrick is cursed by the Wolf Mask and becomes Zen-Aku. He is trapped in a tomb and put to sleep by the Ancient Warriors (his friends). Therefore, they might have been world's very first Power Rangers.
4. In PR Lost Galaxy, on Mirinoi, over a millenium ago, the Quasar Sabers are placed into certain rocks on Mirinoi. So the very first users of the Quasar Sabers might have been the very first Power Rangers.
5. Eric, in PR Time Force, found the Quantium Controller in the present, so it is unlikely that he was the very first Power Ranger. Besides, the Quantium Controller was made by Time Force.
6. The Ancient Mystic Warriors also had powers. And that was before the MMPR era. Maybe the Ancient Mystic Force team was the very first PR team.

From the morphing side:
Just because Zack morphs first, that doesn't mean that he is the world's very first Power Ranger. The morphing sequence could start with either Jason, Kimberly, Trini or even with Billy! Or maybe even with Tommy, if Zordon would originally choose six teenagers to be PR...

From my point of view, the very first MMPR team was the very first PR team ever.
Either them or the very first team of ancient warriors which had any Ranger arsenal at their disposal.