IMHO the best Atari ST retro monitor is the Dell 2001FP flatscreen, with the soundbar attachment if no other speakers are used. It's 4:3 and has low latency so fast scrollers still feel quite smooth, although never as smooth as an analog screen would.
It does all three ST resolutions, but you would need a specially made cable (similar to RGB SCART, but using a VGA connector) for low res and then a switch or a separate cable for high.
Do note: Manufacturing date of the 2001FP is important. More information here: http://www.atari-wiki.com/index.php/Flat_Panel_Displays