I usually go with Apple Store but last year I decided last minute and ordered from Carphone Warehouse the day before release and it actually arrived release day.
When I used to preorder new iPhones it was always through my carrier for the better deals. When it comes through the post it doesn’t matter to me who it comes from as long as the price is good.
In the past couple of years I've noticed Apple's delivery estimates are pushed back further than T-Mobile soon after they start pre-orders. Who knows if they've bumped up supplies for iPhone 11 this year for initial shipments.
I always pre-order/order through the manufacture . As mentioned above, Apple direct has the most available. If their website doesn't crash, then use carrier websites.