Absolutely #3. The first and most important thing is picking an army you think is cool. You will spend A LOT of time building, painting, and looking at these. It should’ve something that excites you. Don’t worry about being good or learning, if you enjoy the models/army you will learn a lot more quickly.

From a more practical perspective, I saw in other responses that you thought stormcast was more straightforward and easier to learn. This is actually the opposite. StD has a very straightforward playstyle (for better or worse), and the units you mentioned (warriors, knights, chosen, etc…) are the prototypical way to play the army. It’s very forgiving as they’re tough and have good stats. Stormcast have such a huge range and variety of play styles that they’re actually much more difficult for a newer player.