There are many different kind of adult chat rooms available, most of which are free, very few require payment in this day and age. To determine what's considered as the “best”, it would vary from person to person and come down to what kind of sex chat experience the individual is seeking.
The old school, popular and in my opinion the best are community free chat rooms. These are cam chat rooms which typically consist of rooms filled with people that have a user name, the ability to free chat as a group with everyone and the option to select individual users from the free chat room list and direct messaging. A lot of these systems also support video chat and the use of mics.
Any style of chatting which has in the more recent years become popular is random chat, or roulette style chat rooms. This is a more anonymous and has no group aspect, you're literally dumped into a conversation with another random individual and literally talk to a stranger.
Pro's of community chat rooms:
Con's of community chat rooms:
Pro's of Random Chat / Chat Roulette style free chat:
Con's of Random Chat / Chat Roulette style free chat:
Other important factors as to what's considered as good free chat rooms are whether they support the features that you're looking for. Do you chat on desktop or mobile, or require the abilities of a free cam? It's hard to find a chat service that has a chat system compatible with all mobile devices and browsers, and in some cases websites will use separate chat instances one for desktop and one for mobile, so when you connect on another device, you might not be in the same environment.
Are you looking for a Niche chat? Some chat sites don't have any specific topics or categories, few have chat rooms for a specific subjects, genders etc. Take free gay chat rooms for example, since it's a subset category, it can be hard to find a free gay chat and even harder to find a chat room that is populated.
Does it support webcams? In the modern age free cam chat is a big thing. Many people are only interested in free cams and require it. Although most free chat rooms support video chat, it's hard to find a respectable one that offers them for mobile phones too.