Again IF you're asking this you do not posses the skill nor resources nor intelligence (no offense this is harder than you think) nor tools to do this.....additionally you will face HUGE criminal charges for doing this, and would probably end up in jail for a few years over a joke. Just give up, you won't be able to pull this off. Again this requires LOADS of computers to do this (you either need a few thousand of your own, or you need to know how to infect a few thousand). One computer can NEVER crash a site through DDoS/DoS...it requires too many for you to pull it off!
-No way to patch a server against an attack
-Works on every type of server
LOIC and HOIC cons:
-Relatively weak alone (many users are needed to take down a site)
-Uses flooding, so can't be used behind a proxy or Tor
#RefRef pros:
-Exceedingly powerful (took down Pastebin in 17 seconds for 42 minutes)
-Doesn't use flooding, so has a low bandwidth requirement and can be used behind proxy or Tor to hide identity
#RefRef cons:
-Very easy to patch against, so only works until the server is patched for the vulnerability
-Very limited range of targets. Only attacks certain servers and only by using a PHP page with a certain type of input
SlowLoris pros:
-Very low on bandwidth, and so can be used behind proxy or Tor
-Works on any Apache server, and the best patches only give about 10% resistance
SlowLoris cons:
-Takes a long time to take a website down
SlowLoris describes itself as a "low bandwidth, yet greedy and poisonous HTTP client".
I haven't used Keep em' Dead or H-Ping enough to know their pros and cons.
Only LOIC and HOIC have GUIs. The others must be run through Terminal (on Linux and Mac) or Command Prompt on Windows.
Good luck!