I would say Cali just because its still technically illegal(even though its not really legal in Amsterdam but much more tolerated) . Amsterdam has had years to perfect their techniques and grow ops while most of the suppliers in Cali are just starting to become mainstream.
Just because the growers are becoming "mainstream" now, doesn't mean they haven't already been growing for years, perfecting their own technique. I mean, when the first dispensary opened up, they weren't selling dirt weed, now were they....?
But, I don't think anybody can really say any one location has the "best" weed, just for the simple fact that each area is different. Every area has the potential to bring something different to the table, ya know? Different climates, techniques, strains, etc, should all be taken into account when even trying to narrow it down to who has the best cannabis. I mean, trying to compare the three locations, would be like... trying to compare who has the best sunlight. "Florida, Australia, or California?" I was just looking at past winners of the Cannabis Cup, and there's a lot of winners coming from the Netherlands. But, does that mean they have the
best cannabis? No. Because other countries have won it as well. I just thought it was worth mentioning.
....I think the only thing you could really compare the three locations to, would be something like... "availability, prices, legality issues, social acceptance, etc." But, that's neither here nor there.
TL;DR: Can't really compare who has "the best." It would be
almost impossible....