For me the Giro is the best. Im a cycling fan. I like watching cycling. Prestige plays a part. The Tour de France is more prestigious, it is our blue ribbon. But prestige is ONLY 1 consideration. The Others would be parcors, calliber of riders, excitement. In calliber of riders the Tour wins big, but personaly for me the calliber of riders is not that important. If a gt has riders who arent the best, then that just means the race will be better because for most of the challengers, it is their best chance. Someones dream will come true. A GT does not lose prestige if a non prestigious rider wins. On the contrary, the rider who wins gains prestige by winning the GT.
In parcors the Giro OWNS the TOur. Terminillo, Gappa, Zoncolan, Kornplatz Mortirolo, Gavia Pass. Thats 6 huge mountains within 30km of a stage finish. COmpare this to 2 - Madeline and Tourmalet in the Tour this year. Take into account that the Zoncolan and Mortirolo are both bigger than anything the TOur has.
Less sprinter stages. A tt doesnt have to be 50km. I prefer 2 exciting 10- 20 km tts rather than 1 long 50k tt. A ttt is always there as well, as are breakaway stages. The less sprinter stages the better.
In excitement the Giro obviously rules. The mountains are more exciting, the sprinters stages are more exciting- breakaways have a chance.
In conclusion the Tour is more prestigious, but its prestige is NOT enough to make up for its lack in excitement.