Not sure what is best or most feature rich, but you can spend between free and high 4 digit numbers on software.
What i do know is that you do not want to do subtitling in Avid.
A standalone tool is way easier, but comes with a learning curve.
(there are a lot of handy tools in a subtitle program that cannot be found in a text editor)
To get the subs into Avid is another story. I hate the subcap effect for final output (it does not look good, is buggy as hell especially on Mac, but it does do the job for screeners on PC.)
AAF would be nice, but not sure what software does this.
My trick is to use a QT with alpha if i need to do in-vision subs.
If it is a one time off, outsource. If you want to dive into it yourself, do have a look at my subtitler:
But do remember it's a craft by itself that needs to be mastered, where the main focus is jiggling with language, not technical stuff.
A beginner can easilly spend a couple of days on a feature and still don't get it right.