Yflyer From United States of America, joined Feb 2007, 1064 posts, RR: 1
Reply 1, posted (6 years 3 months 3 weeks 4 days 12 hours ago) and read 3116 times:
According to the comparison chart on seatguru.com, for domestic economy class it's JetBlue, which ranges from 34"-38" on their A320s. If you want to count United's Economy Plus, it's 36" on the A320 and 757-200.
For international economy, it looks like it's Thai Airways at 36" on the A340-500, also according to the seatguru.com comparison charts, but they don't include every airline.