Do you have any experience as to which mark is the "best"?
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?