Depends on your background / intended purpose:

1. if you are making web apps then Google App Engine / Python is a good choice

2. If you are developing Ipod/ipad/iphone touch apps then you should learn obj C

3. If you are more scientifically inclined and working in bio / Chem field then again I would recommend python.

4. If you are you young and have ample spare time at your hand then learn C.

5. if you are just plain beginner with no clue then I would suggest Ptyhon again.