So far as I know, only Siem Reap Air. Since they have joint ticketing with Bangkok Airways, that's effectively no choice at all. Also, looking at the Siem Reap Air website, there seems to be an announcement about suspension of services - but the link is dead, so you can't get any more information. They only scheduled flights they are booking are internal ones between Siem Reap and Phnom Penh - they must be having problems of some sort. That seems to be confirmed by the Bangkok Air site, that is only booking PG (Bangkok Air operated) flights on the route at the moment.