I took the 552 bus today from the Airport Transportation Center, after having taken the shuttle bus from level 1/West Bus Lobby (near door 15), to On Nut BTS station (so Sukhumvit Soi 77). I think that this bus terminates in Khlong Toei near the Tesco Lotus on Rama IV. Cost was 35 baht, travel time from the ATC to On Nut was ~ 45 minutes. We went via the Bang Na-Trat Road (lower level, access/frontage), until we intersected Sukhumvit Road and then took a right onto it close to the BITEC convention center.

FWIW, the 551 bus is now labeled "Siam Paragon" instead of "Victory Monument", so I have no idea what route that bus follows now. I sort of assume it still goes to Victory Monument then heads south on Phaya Thai road then left onto Rama I to the Siam Paragon Shopping Mall.

There was an AE2 bus waiting but I wanted to try the ATC and the city bus. There were no travelers on the 552 bus, just people leaving work at the airport, but there were some travelers on the shuttle bus from the terminal to the ATC.