Best/ cheapest prepaid data plan in the US?

I'm going to be in the US (LA area) for a month and I wanted to get prepaid data plan, as I want to be able to respond to work emails and so on...

I was wondering which carrier was the best in terms of network coverage, speed, data volume and price.

Since my phone uses a SIM I would't be able to use Verizon, I guess.

Maybe some of you can give me some tips...

Thanks in advance!