For me it's Skaven and the reasons are kinda numerous. I started playing Warhammer originally through the video games, my favorites of which were Mordheim and Vermintide 1/2, which of course heavily focused on the Skaven. Eventually I started playing the Total War serious and fell in love with the play style of the Skaven armies (which I picked out of familiarity due to the other two games.)
Fast forward to the start of this year, my wife and I are walking into our LGS for the first time and there is a huge warhammer scene there (40k but shifting towards AoS) and they were extremely generous to us to help us get into the hobby as they were really craving new blood. Wife got an entire Seraphon (her personal favorite fantasy race) for 300$ (about 5k points worth of collection.) and the head of the store gave me the Skaven Vanguard box on store credit, which I only have to pay back 10$ a month on, on top of which a guy I met at the store brings in his Skaven to supplement my army with so I can play 2k point games, so needless to say, jumped pretty head first into the hobby rather quickly, especially when the scary money barrier just gets kicked down by the kindness of others like that.
Now, as anyone who has played AoS Skaven can tell you, they have a TON of rules and it's hard to memorize, however, I feel in love basically instantly. Despite the old models I adore how they look, the play style is extremely fun and building them out as an army is so incredibly flexible you can do just about anything you can dream up. They feel good, play good and look good (imo.) So, that's why I play Skaven.
Also for anyone interested, specifically, it was the first time I charged a unit of 20 Plague monks, buffed with Rabid-rabid, Death Frenzy and hoarfrost to wound on 2s into an opposing Ogermaw player and absolute devoured his units both on my turn and his that I really began to love them. Glass cannon rats that blow you up and then blow up on the way out is just hilarious to me. Not to mention anything Skyrer is just *Chefs kiss* RNGperfection.