stam for sure.any viking level 1850+ will tell you increase stam since at that level exp required to level will suddenly double
If you have high enough stamina you can let it regenerate during the course of a day and when you log on you'll have a hefty chunk of stamina to throw at battles and bosses. Or if have enough stamina to where you can level up ten or more times before you run out then you can use favor points to recharge and level up indefinately (provided that you have enough opponents you can beat) Once you get your skill point achievements I would reccomend focusing on stamina, attack and health.
