The best Muay Thai gym in LA in my opinion is Sityodtong in Pasadena. The training there is unbelievable and you will get to train with amateurs and pros of all levels.
Here is their website:
www.protraininginc.com
This was the gym that Bas Rutten recommended to me (prior to him opening Legends) and I LOVED my time there. The instruction is top notch and you will have a great workout.
They also have the best boxing I have seen. I learned to BOX there.
There is also Legends however I don't know how the MT is but I will be training there soon. Their website is
www.legendsmma.com .