Actun Chapat and Actun Halal get great reviews. This well written article has some great pictures, and you can get a good feel for Chapat. The Trek Stop in Succotz, along with its butterfly farm, gets a good mention.
"We left Actun Halal behind and headed deeper into the jungle along an ancient Maya causeway toward Actun Chapat, an extensive cave or - perhaps more accurately - cave system with underground lakes, Maya artifacts, and miles yet to be discovered. It was hard to take photos because the moisture in the air obscured the formations if I used a flash; I used my small tripod and took mostly available light photos. Some of the chambers are massive but I wasn't able to document that with a camera."
CLICK HERE for the whole article with lots of photos