You will probably get different opinions on this but i think it depends on what you want to learn and what you already know. I use CorelDraw most of the time but I also have Adobe Illustrator that comes in handy on some projects. I also use Inkscape and Photoshop quite often. It is always nice to have all the tools that you can get because you never know what the graphics world will throw at you.
If you have a limited budget, I would give Inkscape a go. It is Open Source, so available free.
www.inkscape.org/
Just because InkScape is free doesn't mean it is rubbish. There is a lot of good free Open Source software available, Mozilla FireFox is the most well know one - A safer alternative to Internet Explorer:
http://www.mozilla-europe.org/en/firefox/