Best Cheeseburger in LA is Tommy's I agree...
That I have ever had is a place we used to go to in college in Denver called the Cherry Cricket. Its like staring into the face of God and having him smile back at you and say "You are my most precious creation."
if you're in or near Bellflower, try this new place called Fronk's. The burgers are really good, very good quality and flavors. Dont forget to order the sweet potato fries or the onion rings....AWESOME. I give this place a 5 out of 5.
Bad location though, on Alondra between Clark and Lakewood Blvd. Next door to a carniceria, Chapala meat market.
In recent years, the best burger I had was at my uncle's house. He busted out his wood-fired bbq and cooked up some burgers that had the best naturaly smokey flavor ever. Fan-friggin'-tastic!
But alas, he lives in Washington state, and I live in L.A.
I finally got around to trying TK burgers (in Costa Mesa and various OC locations) and they were pretty darn good. Not anything special or amazing or out of this world topping/size/etc, but just a good regular burger. Combined with some excellent onion rings and this is my new favorite fast food burger place.