What browser is best supported by Wordpress.org for editing posts in the visual editor (which I guess is TinyMCE)?
I use Google Chrome and have run into issues which are specific to just this browser. Here's an example bug.
TinyMCE is generally cross-browser compatible. However they provide the best support for IE, Firefox and Safari. I've never ran into problems with my beloved Opera too.
If there are any bugs they're mostly not browser-dependent, but if they are - try Firefox, Safari, Opera, IE - in that order.