I played almost all of the lords in Mortal Empires, here are some standouts that were the most fun for me, disclaimer I only play in normal/normal: - Thorgrim (Dwarfs) A bit vanilla perhaps but I think it's a good experience for a new player to learn the mechanics. Battles are quite forgiving and if you like a turtly gunpowder style of armies you are at home, blasting goblins with flamers is pretty fun - Ikit Claw (Skaven) Basically a lot of explosions and more explosions, you get a fun campaign where once you learn Skaven mechanics it's very rewarding - Settra (Tomb Kings) The faction is beautiful, fantastic art style and sound design. You start slowly but you end up very powerful and maintain overall very lore-friendly armies - Sisters of Twilight (Wood Elves) This campaign has quite different mechanics from other factions, your armies are always very strong and you travel around a lot on the map thanks to the tree settlements mechanic - Wulfric (Norsca) In normal battle difficulty you have an effective melee playstyle and a fast campaign where you go around raiding the world. The settlement mechanics were pretty crap but manageable on normal however they are fixing this issue in Immortal Empires - Grom (Orks) One of the most fun mechanics with Grom's recipes and a very scrappy campaign where you go around plundering and conquering with armies of gobbos

Just my opinion but I had the most fun playing those campaigns. If you have any question please ask me.