It'll be better than the margo victory, obviously. I'd actually place it right between Bradley and Cotto.
It depends on what Spence goes on to do. If he goes on to beat Crawford and establish himself as a top 3 p4p fighter, yes. MAB was top 3 p4p and Pac, an underdog, went up in weight and demolished him. Pretty hard to beat that.
If it's a close fight that Pac edges I'd still rate the Barrera win over beating Spence at 42. The way he dismantled a great like Barrera was just breath taking. But if Pac beats Spence convincingly or shockingly stops him then perhaps it might rank as higher simply because he hasn't looked like the overwhelming dominant force he was in his prime for a long time and to turn back the clock against this level of opponent at this age has to count for something. But I doubt Pac will dominate even if he wins, I expect this to be a hard fought contest.
It would be an all time great win in terms of overcoming disadvantages that weren't self imposed (as in, all the fighters who have come back from prison or rehab or the couch). The age, the size, the activity, the home field advantage, it's going to take a miracle.
Even Bradley is a better and more proven win... It's the best win by a 42 year old, but really Spence is not more accomplished than Bradley or Cotto at WW, and isn't in the league of Marquez Barrera and Morales.
Probably would be his most historic win? Set even more records. Oldest WW champ? Oldest Asian world champ? Oldest Filipino world champ? First unified Asian champ? First unified Filipino champ? First unified WW Asian champ? First unified WW Filipino champ? First man to beat Spence? First Asian Nth-time world champ? First Filipino Nth-time world champ? Would it be a lineal title? What else?