A friend was told by the staff while dealing with mouth sores in hospital (post bone-marrow transplant, so really, really needed to keep infections under control) that the best mouthwash on the UK market is Corsodyl as it actually has proper antiseptics rather than just an alcohol base - I'm now using it myself, but it's noticeably not as nice a taste as others I've used in the past. A quick google tells me Corsodyl has chlorhexidine gluconate as an active ingredient, and that the US equivalents are peridex or perioguard, but possibly prescription only.