All TOC's have their flaws at Revenue protection from my observations, FCC seem to have the strictest RPI's but hit and miss with many unmanned stations/no guard on train to collect fares/open barriers!
Whilst Southern and SWT are good at manning Barriers but again not all trains have commercial Guards and the prescence of RPI's is hit and miss. Also I find especially my TOC's RPI's are far too lenient and just try to work as little as possible so not very efficient!
FGW are interesting as they are starting to Barrier more stations, their major ones, but more is still needed in my opinion, even if only part time staffed I reckon there is good Revenue to collect from some of their busier unbarriered stations. Down south their HST runs include frequent stopping and with such long trains it can be difficult for the Guard to collect revenue from all carriages let alone after every stop!