Giro has the potential to be great depending on what legendary passes they use and how they use it (clearly the best geography of the 3 IMO) and a very average field as of late with lots of weather related problems which put a damper on things, Tour is the Tour and has recently been the best GT as well with great battles and pretty good routes (especially 2022 stands out as the best route, maybe ever, in the race), while the Vuelta has the uncertainty, a great field on average and a route which differentiates a lot to the two more tradtional GTs with with lots of MTFs spreadout through the race and close to nothing for the sprinters. On average that makes for a better race.
In terms of watchability, its quite clear. Tour is on top, but the Vuelta is not that far of while the Giro is slumping massively.
1. Tour de France
2. Vuelta a Espana
3. Giro d' Italia
In terms of prestige, right now, I think the Vuelta is making a bid to become number 2 as well. I already consider it the second most prestigious GT due to the field which mostly comes down to the calender since there's almost nothing left for stage racers after Tour de France, but I realize that most people are probably still stuck in their ways. For me its quite obvious though that in terms of prestige and how hard it is to win, the Vuelta is 2nd.
Prestige:
1. Tour de France
2. Vuelta
3. Giro d' Italia