The Wandering Spirit.
The answer changes for the regular Spirit and the Wandering.
Wandering: Lore by far.
Spirit: Hard to say, depends on what you're looking for or building your character around.
Specifically lore as a wandering spirit. Arcane enlightenment is far better that way than as a permanent, fixed hex.
As permanent spirits? Battle would be tops for melee, ancestor is well-balanced and can serve several purposes well, life is good for a healer, dark tapestry for a summoner - as with many things which is best depends on what you want to do.
Your wandering hex can be from your primary or wandering spirit. The lore shaman has the advantage of getting the wandering arcane enlightenment while having the freedom to change his wandering spirit every day to suit the situation. Most other shaman are shackled to one wandering spirit, which kind of defeats the purpose of having a wandering spirit.
I'm partial to the Waves spirit. While on its own it is not objectively better than others, combining Crashing Waves with any spell to force an additional saving throw or be knocked prone (this starts with [water] type spells which any elemental spell can be made such with the use of the Benthic Spell metamagic feat, and then extends to any damage spell later on)... Well, shenanigans can occur.
Also, Fluid Magic is as useful as whichever spirit magic you have access to with your other Spirit chosen. Get yourself a conductive weapon and abuse the knockback ability of Wave Strike. Get creative and manage to pick up a means to spellstrike at the same time and you are golden. Hit, knockback, save or be prone. Use a melee weapon that uses a touch attack (like a whip) and even better.
I built a Shaman with the Waves Spirit as my main, and the Life spirit as my Wandering Spirit. I was QUITE effective in my chosen role. It was quite easy to adapt the character to his role. The only feats I needed were Benthic Spell and Extra Hex. Metamagic reduction traits were helpful for Produce Flames and Pale Flame.
Nothing like checking around 2d6+5 ranged touch attacks that dealt straight bludgeoning damage if I wanted them to do so. Even better that I could use them as melee touch attacks.
I am a big fan of waves too, I have always wanted to take it on a Speaker For the Past to grab some Ancenstor&Time Revelations
You can certainly change your wandering spirit every day. It's just that that one particular hex is more powerful than everything you can get from some shaman spirits. Lore (as your primary spirit) is an example of such a spirit, the spirit magic spells are seldom useful until you reach character level 15 (legend lore as a 4th level spell is interesting but not useful that often, the others are worse), the spirit abilities aren't great, the other hexes are bad.