Illustrators name is a bit misleading. You can make illustrations in any graphics program but if you are specifically looking for vector art then Illustrator is your thing.
I'd say GIMP if you want something free. Sai is affordable. Manga Studio is mid range, mainly inking and comic related. 5 has an extremely expanded color support. Photoshop has less smooth line tools but is crazy versatile. Also expensive.
Also check out Painter and Sketchbook Pro.
It depends what works for you and what you want to work in.