Generally speaking I think Stormcast are probably the most beginner friendly, as the poster boy faction they're going to get new and interesting models with every starter set, metallic paint schemes are relatively easy (and Stormcast still have bare face options, cloth and other embellishments meaning you'll still be learning how to do those things) and you're basically guaranteed not to languish without a book update as the editions of the game roll on. The downside to that, of course, is that because stormcast will constantly be updated they'll always be the benchmark other armies are measured against (and will therefore always have an answer to) and because they're constantly being updated you're going to find certain units are effectively replaced by others over time.
As for the armies you mentioned, I have a Sylvaneth army that I rather like and I do think they can be relatively easy to paint as they actually benefit from the variation that easy techniques like drybrushing can cause. They have a particular emphasis on board control that might be tricky to pick up if they're your first army, but they were mine in AoS so it's not all bad.
Of the three, Seraphon are probably the best option for learning how to paint 'normally' as they have elements of skin, metal, cloth and other things with varied textures so you'll be learning how to paint those different things effectively as you go. Mechanically I understand they're pretty dependant on the summoning gimmick and are apparently hated for it.
Nighthaunt were introduced as something distinct from Legion of Nagash due to the former coming out after the latter, though I understand LoN can take Nighthaunt units there is a difference in how you'd utilize them - need a Death player to sound off on exactly how though.
IMO, for your very first army I think the best way to go is to pick something you like the look of and run with it. Learn how the game is played and learn how you can improve your army within those rules, then when you have a better grasp of how to paint how to play, and what you want your army to do, look over the options again and build a second army based on what you've learned.
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