. Determining the criteria in flight choice primarily depends on what you are willing or unwilling to tolerate, put up with, and the level of your comfort zone. Apart from the obvious, ie safety record, I am not that picky or discriminating. Coming to London yesterday the layover in Seattle was over 4 hrs, the other choice was Phoenix. I picked Sea-Tac anyway.
Flying to Europe I look always for a direct flight (10 to 11 hrs) at the right price, agreeable depart times from SFO or OAK, landing in the morning ca noon or prior in Frankfurt FRA, Paris CDG, or London LHR, unless the price difference is so marked to be in my favor. Then one change per leg max, also depends where.
The ideal dep time from here 2 to 4 pm; from there returning be it FRA, London or Paris CDG likewise. Going over my choices have been (so far) BA, United, (on time or late), Air France, Lufthansa,....will consider Air Canada, Virgin, Norwegian as future possibilities
If the return is back to SFO, I don't care when I get in, be 4pm or midnight, if it's OAK, the latest is ca. 8 pm.