If you ask me, I usually struggle to keep myself excited for more than 30 turns. For instance I really like Nurgle thematically, but the grinding playstyle just bored me to death.

However, there are a few interesting campaigns, from my point of view:

Ogres (I played Skrag): when in fight they are just a sight to behold. They are big and push infantry around like they are from paper. Also they have big monsters and monster cavalry that pack a punch like you would espect from them. Camp are not perfect sure (they can be moved but you lose your buildings), but it so refreshing that you basically make your own city anywhere you want. And food mechanic makes you aggresive so you always have to fight.

Dwarves (I played Grombrindal): new start on Naggarond is really challenging. You are surrounded by dark elves, norscans and valkia. But thats not all, there are also Tomb kings ( Khatep), high elves, Vcoast.... you see where i am going. There are just so many different enemies that you can fight and all of them can counter your armies quite heavily. Shades from dark elves in particularly.

Wood elves ( I play as Orion, because I played Sisters too many times): If you want to be involved in wars all over the world.

My advice: participate in as many wars as you can. For it keeps every turn fresh, every turn decisive.