Most important tools

1. brain, brain, brain -- I couldn’t stress it out further or "a fool with a tool is still a fool"
2. fundamentals, you have to certain about network and system fundamentals
3. listen to your client
4. LookAtYourLogs
5. ping
6. telnet
7. traceroute
8. nmap
9. nessus
10. perl

There is no magic problem solving tool. Most of the time i use very simple tools and fundamental know how.