By quality in my opinion based on owning a few shirts from each:
1) tyrwhitt sea island
Tie for 2) and 3) Brooke's brothers and standard/non-iron Tyrwhitt
4) tm Lewin
Mind you the margins are verrrrrry slim. The Tyrwhitt "tailor fit" shirts fit me best and I like their styles better so I go with those. They come with nice brass collar stays and I notice much less loose threading after wearing and washing verse my TM Lewins. Not to mention CT customer service is far superior to TM Lewin who sent me last package without tracking. After 2 weeks with no delivery they basically said "Sorry, give it another month". When I had a problem with some CT shirts I returned I got all kinds of emails from customer service and Nick Wheeler's office and to fox it they altered and had the shirts on my doorstep (I'm in the US) in 5 business days.
*i apologize for any typos/awkwardness I'm typing this on my tiny phone keyboard.