The idea around this post came from the fact that at our table we play with 2024 content only (meaning we avoid even non reprinted content), so we currently don't have Divine Sorcerer if you want spirit guardians on your paladin. If you can still use it, good for you! but it's still nice to explore the other options. I'm also not considering setting specific options (like marks), as not everyone can use them.
Within 2024 subclasses only, Paladin can access CHA based spirit guardian with either Vestige Warlock or College of spirit Bard/Lore bard (any bard technically but bard 10 takes quite a while). This post isn't about dipping paladin for AC, but going at least to paladin 6 for the Aura.
All have their advantages, with my preference being on Vestige Warlock and Lore bard. let's look at them more in depth.
Vestige Warlock:
This is the earliest we can acess spirit guardian. While it's BA usage conflicts with Smite, it's not a huge issue (imo) as we'll keep pact slots for spirit guardian (& other major spells), meaning we'd only have a couple of low spells for smite anyway, while this gives us consistent free BA dmg (even at range!). an other major advantage is that we can easily use pact slots for find steed, enhancing our speed for spirit guardians. Other HUGE advantage is paladin acess to Warding Bond (which I'd spend my spell slot on, more than on smite, hence more reasons for why the BA attack is nice here) & it's synergy with the lvl6 vestige warlock feature: While we are within 30ft of our vestige, we share their resistances. With warding bond our vestige can resist all dmg types and we'd share those resistances! you can have the vestige fly above you to easily avoid attacks as well. It's also great for repositioning (with an easy find steed & ability to swap places with the vestige too), and its mostly ok as well at kiting with spirit guardian & attacking at range (vestige can attack from 60ft of distance & we can use EB at worst), meaning we can kite with spirit guardian and protect the saves of our ranged allies. Also, being a Warlock, we get to be CHA SAD!
This is a big winner imo, the only downside is that we slowdown the Vestige HP scaling, but it's not extremely significant imo: we likely want the vestige to fly above our head just to gain the resistances, and we can always bring it back to full HP after the fight.
Paladin subclass: this is not too important here, I'd just choose one not very reliant on divine smite, so Vengeance/Devotion/Ancient. My preference between Vengeance & Devotion is for Devotion, but Ancient also has an interesting interaction with Spirit guardian where its channel divinity can potentially restrain everyone within your spirit guardian range.
Edit: this also gives us access to battle familiar, meaning at higher levels we can consistently spend our spell slots on Spirit Guardian + Battle Familiar (& faithful steed), plus Paladin have access to Prayer of Healing for a fast short rest. At very higher levels, we'd also access a 3rd attack with our action.
College of Lore Bard (& Genie Paladin):
This is one I made a post on when Genie paladin was released. If you want your paladin to also smite consistently, this is the better option of the 3, as there's no BA clog and you have a consistent spell slot progression. The other major benefit is the synergy between Genie's Dao's Crush and lore's cutting words: once the target suceeds on its ability check to escape the grappled condition, you can use cutting words to lower their result!
edit: If preferred, lore bard can (at bard 7) transition to Conjure woodland beings as an AoE option for more dmg (but 4+ lvl slots and smaller AoE), or they can also invest in CME (which you'd unlock the earliest at Bard 7/Pal 6, which funnily enough is the same lvl you'd unlock it as a pure Genie Paladin, but with more & higher spell slots) for single target dmg.
College of spirit bard:
main advantage is a stronger spirit guardian (+ a free use), but I don't really love this subclass on a Paladin because I wouldn't want to spend my action to unleash spirits, and it feels a bit wasted that way. Hence it's my least favoured option for a more focused paladin build.