Cat shampoo?

I'm here to ask for recommendations for a shampoo for my cats. They're just over a year old and I know nothing about cat shampoos. There are so specific reason other than to just keep them clean once a month. They are indoor cats that only go out to the garage, an upstairs balcony and a gated patio with concrete floor.
I've looked at the local pet places and I'm totally confused by all the kinds and brands.

Thanks.

With Halo I use a shampoo that was actually reommended for our Westie(they hv sensitive skin). It is an all natural shampoo that can be used on dogs, cats, ferrets, etc. It's made by Kenic and is called Kalaya Emu Oil Pet Shampoo. I found it online at jefferspet.com. It made a HUGE difference on their coats. i'm sure there are other better options out there. I used it cause I bought a gallon and had plenty.