It all depends on how you're playing.
Solo - If Mobs rarely hit you and/or you have lots of hp left after a fight ---> Improve DPS by upgrading Wound, Forced Swing, Mangle, Knee break in that order
Group - If party members can take aggro from you --> Anger, hold the line (if in battles where you can stand still often) taunting blow,
If you hold aggro or are solo but you're getting hammered and/or your cleric is always out of mana - soldier's stance, battle tactics
The wild card options are assault (only if fighting multiples and without an enchanter around), and the power draining line which is really most useful against casters. Don't do shield bash.
Within these recommendations modify my personal preferences by the frequency you use the skills - don't use it = don't pgrade it. All these are just my reasoned opinions (21 Guardian) and I'd love to hear what others think.