Sai is great with brush but doesn't have text tools, I've heard rumours about it having problems with file corrupting. I have not tried it personally.
Manga Studio is great, has awesome brush feel and blending tools work fabulously but lacks abilities with text tools. I use it.
Photoshop is versatile and has great text tools, very customisable brushes, but lacks in brush sensitivity and blending tools are bit awkward. I use this as well.
Have not used gimp or heard anything much about it.
I've seen fire alpaca in use and it seems very valid tool to start with.
Pen and paper are really good, but I often find it irritating missing the undo button