The best OS I want:
1. Runs Everything!
2. No Bugs!
3. Cannot be hacked!
4. Does not slow down!
5. Fast!
6. Cool Design, but not too complicated!
7. Free!
Well 1, 2, and 3 are literally impossible for no matter what OS you are using. Although, Linux is much more secure than Windows and OS X. Partly because it's open sourced but being open means it can be easier for people to exploit the software.
If you run Linux, it will not slow down. Period. Only the necessary daemons are loaded during the startup and the ext filesystem rarely fragments. Whether or not you have the speediest OS depends on what desktop environment you choose. If you want something fast, look into XFCE or LXDE. If you want something that looks nicer but uses a few more resources, look at KDE or Gnome.