I have a britax roundabout now for my 1YO. Its been a tight squeeze on a few of our flights be we always manage

If you could have one car seat for JUST flying (mainly international) what car seat would you use? And why did you pick it? (lightest, smallest, fold-able, etc)
I ask this because most places we go we do not have our own car and we don't normally travel in cars (tuk-tuks, baht buses, buses, trains, etc) so we never use the car seat for a car, its only flying. We have a CARES unit which is great when he is awake but is lacking while he is sleeping (he is only 22lbs now i'm sure it will get better though).