If its a new monster, I like to open up with a Com/Rav/Sab then as soon as deprotect and deshell pop on the monster I switch to Com/Rav/Rav in hopes of a quick win
If hes doing a fair bit of dmg, but not too fast I switch to Med/Rav/Med. If hes a fast monster then Med/Med/Med or Med/Syn/Med.
My party is usually Lightning, Hope and Van. If I need a Sen then I switch Hope for Snow.
Edit: I now use Light/Fang/Hope.
Fang is the best COM and Light is the best RAV + Hope is the best healer and second best RAV so they make a good team. Not to mention that gives you a good SAB, SYN and SEN when you need 'em too.