Feels like Glasgow is slowly losing that title.
Its clear that Edinburgh is easily the number 1 airport in Scotland now. It has always been strong to the European cities, but with this potential Norwegian expansion especially to as far as the US west coast, its a major threat to Glasgow's claim. Its slightly lagged in a way behind GLA for long haul because of the shorter run way and the fact likes of Orlando, Vegas and the Caribbean are popular with Glasgow holiday makers. Glasgow will still be very strong on the sunshine routes as it always has especially with Ryanair now beginning to operate more leisure routes from GLA.
It cannot touch EDI for inbound tourism however, and also GLA is terribly located. Numerous people the GLA catchment in places like North Lanarkshire find it much easier to travel to EDI being on the west of Edinburgh and avoid the GLA rush. Glasgow can't maintain routes to places like Frankfurt, Madrid, Copenhagen and so on. They may only be 50 miles apart but the clientele attracted by both are very very different in my opinion.
Glasgow could probably maintain services to the likes of Boston, but its finding an operator willing to take the risk. The west coast for me though, absolutely no chance. There isn't the catchment, the business community or the tourism in Glasgow to support a route to a place like LA or SF.
As for Aberdeen, that is dying a death thanks to the oil industry. Expansion there? Forget it.