It all depends actually. By reputation and standards itself, the most sought after is Halliburton from what i hear.
As for the big blue...they got a reputation for hire, train and fire their employees at a blink of an eye.
The big red has quite a good rep too.

My apologies....the big blue a.k.a Schlumberger. the big red a.k.a. Weatherford.

Your degree level gives u an edge on your salary. it does make a difference...
It's best if there's an opening stated in their website. or search for oil/gas careers website.

Working offshore is hard, I heard must work 48 hours non stop.

First u tell other people....and then u said it yourself! 

Mind you...there are peeps who works for 40hrs or more straight...with very little rest in between. If you don't know...don't say anything.

I know because i'm a field engineer.