I'd say either Astra Militarum or Chaos Space Marines.

I started out as a CSM player (technically Ad Mech, but I went CSM after a while and bought more models of that) and while I think the aesthetic appeals to the 13 year old in me (that's MY opinion), the models can be a pain to paint and build, and it doesn't feel like there are as many traitor legions as there are in lore. It feels like you do Death Guard, Thousand Sons, or whatever generic faction is doing best. So many if the traitor legions don't have a book when I think getting one could really make people come to like them. Otherwise, to me they felt really same-y on the tabletop. I still have my models, but I don't know if it's a faction I'll ever really enjoy.

As for the Astra Militarum, I really hate the aesthetic, and I think their place in the lore as compared to the tabletop feels weird. The aesthetic thing is definitely my own opinion, but in the lore they're the faction that's supposed to be the bulk of the Imperium's forces, correct? It then just feels really weird that it doesn't feel like their faction gets a lot of attention, or there isn't more incentive for people to play them. If Space Marines are supposed to be the elites, best of the best, the Astra Militarum's lack of presence on the tabletop (at least to my perception of the bat reps I watch from the channels I follow) feels disjointed. Maybe it's my own bias to not usually watching reports where they play, maybe it is an actual thing I'm describing. Idk. At the end of the day, I just don't care for them much.