Well, it depends solely on what form it's in, seeing as Origin Forme Giratina is an offensive beast, but Altered Forme Giratina is more of a Tank. I'll just a give a moveset for both. :D
ORIGIN FORME (Seeing as you're playing Platinum, it's probably the forme you're using.):
Shadow Claw (STAB, used to hit other Ghosts.)
Outrage (STAB, deals with other Dragons)
Stone Edge (Used to deal with Ice-types, along with coverage against Bug, Fire, and Flying-types.)
Silver Wind (Weak Base Power, but it may boost all stats. Also provides coverage against Grass, Dark, and the rare Psychic-type with good Defense.)
ALTERED FORME:
Thunder Wave/Will-o-Wisp (Use one of these to cripple the opponent. It's your choice, regarding which one.)
Rest (With Giratina's enormous Bulk, Rest is fairly safe to use.)
Outrage (Same as above.)
Shadow Claw (Same as above.)
There's lots you can do with Giratina, because of its bulk, so there's lots of choices and movsets for Giratina to utilize. =)