Southern's customer satisfaction has plummeted on everything from punctuality to reliability and seat availability.
According to Which?'s annual rail survey, the train operator recorded its lowest ever score of 21 per cent customer satisfaction, down from 44 per cent last year.
It received one star ratings for seat availability, frequency, punctuality, reliability and value for money. Southern passengers have suffered months of disruption as an ongoing row over the role of the guard has led to a series of strikes.
Southeastern didn't fare much better with a rating of 31 per cent, followed by Thameslink and Great Northern at 32 per cent. The best-rated operator was Merseyrail with a customer score of 72 per cent, followed by Virgin Trains West Coast in second.
Vickie Sheriff, Which? director of campaigns and communications, said: "Though Southern have performed particularly badly this year, the whole sector is continually failing passengers. Overcrowding, delays, short trains, carriages in poor condition - many services aren't providing even the basics."
A spokesperson for Southern parent company Govia Thameslink Railway (GTR) said: "Performance is unacceptable and we're sincerely sorry. Our passengers deserve better and, together with Network Rail and its £300m funding package to improve track signalling and overall performance, we're working hard to improve the service."
The £300m funding package will aim to reduce delays on both Southern and Thameslink, funding the replacement of old tracks, better drainage and structural repairs in tunnels. There will also be work undertaken to prevent lorries striking bridges, which is another cause of delays.
The GTR spokesperson added: "This survey inevitably reflects the significant impact of the wholly unjustified industrial action being taken by Aslef and the RMT. We have made comprehensive offers to resolve this."
Jacqueline Starr, managing director customer experience, at the Rail Delivery Group, which represents train operators and Network Rail, said: "It is not true that the whole railway is failing passengers, nor that operators are failing to comply with the law - but train operators know they must do better and we're sorry when customers don't get the service they expect, including those affected by strikes."
She added that huge growth in passenger numbers meant the railway "is full in many places" and rail firms were working together to deliver "more than £50bn of improvements".
Rail companies rated
1. Merseyrail - Customer score: 72 per cent
2. Virgin Trains West Coast - Customer score: 69 per cent
3. East Midlands Trains - Customer score: 67 per cent
4. Chiltern Railways - Customer score: 62 per cent
5. Translink NI Railways - Customer score: 58 per cent
6. Virgin Trains East Coast - Customer score: 54 per cent
7. Arriva Trains Wales - Customer score: 53 per cent
8. CrossCountry Trains - Customer score: 53 per cent
9. C2C - Customer score: 52 per cent
10. London Overground - Customer score: 52 percent
11. London Midland Trains - Customer score: 51 per cent
12. Scotrail - Customer score: 50 per cent
13. Northern Rail - Customer score: 47 per cent
14. TransPennine Express - Customer score: 47 per cent
15. TfL Rail - Customer score: 45 per cent
16. Great Western Railway - Customer score: 42 per cent
17. South West Trains - Customer score: 36 per cent
18. (Abellio) Greater Anglia - Customer score: 35 per cent
19. Thameslink and Great Northern - Customer score: 32 per cent
20. Southeastern - Customer score: 31 per cent
21. Southern - Customer score: 21 per cent