If it's the latter, then no doubt, it was Raghib "Rocket" Ismail and Tim Brown, with Golden Tate coming in third.
Rocket could single handedly change the game. You had to account for him all of the time, and double cover him, or else he'd easily break free. As a punt and kickoff returner, he had no equal. He was also a very crafty downfield blocker.
His mere presence meant that teams were rarely able to play an 8 or 9 man front against us, without suffering serious consequences. In 1990, Rodney Culver and Ricky Watters had very nice seasons at the RB positions, because of that excellent offensive line, but what's often overlooked, is the fact that Rocket (and Tony Smith) spread the field vertically for us, often times taking that strong safety out of the box.
If you look at the two regular season losses to Stanford and Penn State, it wasn't a mere coincidence that after Rocket left those games with deep thigh bruises, that the opposing teams were able to crowd the line with that nine man front, and stop our running game cold. Of course, Ricky Watters and his butterfingers did more damage to us during the Stanford game with his two fumbled punt returns...
Now, in terms of pure receivers, it's debatable, but I would say Golden Tate and Jeff Samardzija. Both of those guys were bona fide #1 receivers who could make some truly amazing catches, hand hands of glue, and were two of the toughest hombres to play the game at that position.
Michael Floyd and Will Fuller would be the next two on my list.