Don't know who to avoid, but for new properties Sansiri, Land & House, Perfect Property etc all seem to have a half decent reputation. Pruksa a bit cheaper/lesser build quality.
https://www.sansiri.com/en/
https://www.lh.co.th/en EDIT - thai version seems to work better:
https://www.lh.co.th/th
https://www.pf.co.th/en/home
https://www.pruksa.com/
I find fazwaz a pain to use:
https://www.fazwaz.com/
Hipflat a lot easier:
https://www.hipflat.co.th/en/
From looking over the last few years everything online is 20%+ over realistic prices.