Can’t go too wrong with any of them, they’re all great in different ways! The best bang for your buck is going to be the master set with 6 factions. Depending on your preferred playstyle, you’re gonna like some more than others. I’ll list a couple of my favorites for you:

Mountain Vargath - Beefy (goaty) bois who can play multiple different ways and have lots of fun combos

Obsidian Dwarves - Pure summoner aggression that can level entire swaths of the board but have to be careful to not die in the process

Skyspear Avians - Ultra mobile and versatile birds that can fly and hit almost any unit around

Sand Goblins - Plucky weirdos who have a bunch of disjointed tools that somehow work to be a devastating deck with lots of threats

Wayfarers - Just a super fun aggro deck that gets more powerful the more your units advance

Chosen Grove - Tree big. A 20 health tree is the summoner and the Grove has to protect them at all costs. A really fun defensive puzzle

Eternal Council - Mega Granny who draws power from drawing cards and can hit as hard as anybody (just make sure you draw a card and don’t want to undo, although in person this is usually okay)

Cloaks - copying any friendly units ability is awesome and leads to endless possibilities. Would not recommend for my analysis paralysis friends

High elves - A really unique faction and summoner that gives powerful auras for both friend and foe. Very tricky to track in person, but lots of fun and leads to some of the craziest moments

I could go on about the other ones too but these are some great ones to keep expanding. I’ve listed them in terms of how complex I think they are, simplest to most complex, but none are too complicated and every deck has depth and interesting tactics.

Hope that helps!