Which is it. Is it karate, boxing, kickboxing, maui thai.. etc.

depends there is no 1 form that is "better" then the rest. Wing chun 4 one has great hand tech but no kicks .. it depends on what you are good at and what you use

boxers tend to be the toughest because of all the sparring

Karate is terrible - I can tell you that from experience.

Boxers are tough and can trade with anyone for a long time so I'll say boxing.

There isnt one that is best. Boxing for just punches obviously and Thi boxing for ya leg kicks elbows and knees. Both of these are effective and they work unlike karate which uses hyperthetical situations instead of teaching u how to fight. Do both if u want to be really good at stand up.

thai boxing

Definately Thai boxing. Decent hand skills, the clinch, elbows and knees which are easily more devestating then punches.

Thai boxing.

No doubt. Uses all parts of the body, and is deadly efficient. No 'Forms' or 'Kata' bullshit, just straight up knee you in the face kinda thing.

Thai Boxing or Kickboxing with an emphasis on Boxing.