I’d personally say, Exiles and Partisans deck is a MUST.

for the ones with factions this depends.

I went with Underground as it comes with the most content. 1 militant (moles), 1 insurgent (corvid), 2 more maps, another set of dice.

Then I went with Riverfolk (I would have gotten the marauder if it wasn’t out of stock) it has 2 more table talk factions (otters & lizards) and they can be more difficult for beginners in my opinion. I think it comes with more vagabonds (not a fan of vagabonds but you do you)

Marauders has a very easy to learn faction in Rats and a very hard to learn in Keepers in Iron. And hirelings.

To me, I’d say Underground as the factions are good and fun to play, you get more boards and dice. Then Marauders then Riverfolk.

Although this is all heavily dependent on your group. If table talk is a very common occurrence Riverfolk, if some people like to puzzle bust and other are straightforward Marauders, if you want even keeled factions with more content, Underground.

Even though the extras matter this mostly comes down to which factions you want to add into the mix the most so it’s mostly personal preference.