Hello.
SCMDraft 2+Classic Trigedit=Best combo.
SCMDraft 2 is an editor, and trigedit is a plugin.
You could use StarForge, but it's buggy (or so I hear) and requires syntax triggering.
X-tra Editor is a modded version of StarEdit, the editor that comes with StarCraft. It's easier to use than the other ones, but has less options.
You should probably start wtih Xtra, since it's the simplest next to StarEdit.
Xtra + SCMDraft 2 = Sex. Period.
I hear StarForge isn't bad, but I prefer clicking my Triggers rather than typing (plus there are a lot of scary stories of people losing hours of works due to StarForge).
Def. start with Xtra Editor.