I see we are debating terminology again. What count as spirits? Between how they are mechanically defined in d20-based books (including some 3pp Pathfinder products), it seems like:
incorporeal undead (spirits of the dead/ancestors)
fey and elementals (spirits of the land/nature)
And then other stuff is up for grabs, depending on whether you want to count astral entities projecting, non-undead that are naturally incorporeal, outsiders, or whatever, plus anything specifically defined as a spirit that may not fit these other categories, for whatever reason (I could see Princess Mononoke style magical beasts being considered spirits, especially with some of the comparisons in fluff I've seen between shaman spirits and kami in discussions).
In terms of stories outside D&D, whether fiction or mythology, it seems pretty frequent, if not universal, that spirits are basically ghosts/ancestors and land/elemental creatures or god-like representations of animals.
If we want to preserve that flavor, there does need to be some hybridizing in the spell list, whether it amounts to giving one class's and additions from another, or whatever. I think witch would be the closest to covering it without making a new list or needing to mash together cleric and druid - because it is a spell list that has already been hybridized plus it includes some of its own elements. If it's between druid and cleric spells, I think druid is the best as a thematic base and then the "spirit" spells that people believe are necessary but lacking from druids can come from clerics/oracles.
Heals, buffs, debuffs, wards, weather, interaction with other elements, divination, psychology/enchantment/mind-based effects, inspiration/knowledge, language/poetry and/or literacy/writing, some offense - whether derived from another category or separately, and shapechanging (plenty of shamanic material deals with physical or spiritual shapechanging) probably cover a good portion of a shaman's kit. In an animist worldview, potentially anything could have a spirit, so there is really a multitude of possible spirit types out there, which means a shaman could theoretically do a wealth of things, but there is a theme/flavor to how they do it and that usually focuses around these ideas.
edit: cut a large chunk about flavor issues for space/relevance.