Few of them seem to post much anymore.
People who helped myself and many others over the years with either direct advice or well explained posts and articles include (but not limited to): Screwtop, Butters, ///, Kai, Ross, Pistol Patch, Thirsty Boy, MHB, DrK (when he's not bagging no-chill or writing cryptic crossword puzzles), Brewer Pete, Airgead, Nige62, Domonsura, Tony, Batz, Dent, Wessmith, Zwickel, Lez the weizguy, Dr Smurto, Rdjevun, Big-Fridge.
Kai troester posts here very ocassionally and his advice would be worthy of consideration in my book. I'd also take brun'gard seriously, especially in relation to water.
From before my time but posts worth reading: Postmodern, Pint of Lager.
Others who don't post loads of advice but I consider them as people who know what they are doing, others I would have missed. Mainly people who brew a lot and seem to consider the whys and wherefores critically. People who accrue brewing knowledge and do their best to put it into practice so they can advise from both a theoretical and practical perspective whenever possible. People who will consider alternative ways of doing things, are respectful of new brewers and realise not every beer has been brewed, not every method tested and not one way is right. Add the latter to people who know their arse from their elbow and you just about have it.
None of the above posters (nor any poster) would I suggest listening to without a critical mind. Find out the info, consider it, put it into practice, see what works for your brewing.
Plenty of people I haven't listed who will advise from their depth of experience in a polite, measured manner and are well worth discussing stuff with. Felten, Felon, warra, bribie all good.