The favored mark (by far) is currently the Mark of Nurgle, which gives +1 toughness. It's a foolproof mark that gives you a little advantage over basically all armies and setups you meet out there.
That's not to say the others are bad though. I am unaware of how much you know of the gamerules at this stage as you haven't played the game yet - Have you studied the gamerules and your codex, as in you units capabilities?
Thanks, I'll keep MoN in mind ;)
I've read through the rulebook once, and I played the game some 10 years ago so I know the general flow of a game. I know there's been rule changes and all that but I grasp the general concept well.
I've read up on the codex as well, at least all the fluffy stuff. I haven't had a chance to sit down and delve into the unit stats and wargear options etc. yet.