As others have mentioned, Verizon has the best coverage in LA (and several other cities). But you get what you pay for. Id say a good 25 to 30 percent higher than carriers like T-Mobile.
I had Verizon for years when I had an old unlimited plan. With my kids growing up, I needed a plan with unlimited text and moderate data, and T-mobile was the clear winner (3 lines for $120). Verizon wanted closer to $200.
I too was surprised at T-mobile's iffy coverage at LAX. Mostly 4G but not LTE on the departures level, and 3G down on the arrivals level. I had to go to the top of one of the parking garages near T-4 to get reliable LTE. Very surprising.
The only other place between LA and San Diego where I noted degraded T-mobile coverage on my drives was near Laguna Niguel and San Juan Capistrano along I-5.