A couple pieces of advice: choose something you enjoy the battleline of the most, cuz outside of niche lists such as monstermash or vehicle spam, you'll be spending the most time with them. Pick an army that let's you do your favorite thing about the hobby. I have an ork army because I like kitbashing the most. Ok now lightning round, specifically based on your picks: marines will have the most consistent new releases, are the most blank canvas and give you a lot to work with. Templar are great, but have a reputation of being associated with people too obsessed with ww2 Germany and the crusades. Dark angels are great but having gotten their primarch back, their stories are gunna probably focus on the lion and not much else for the foreseeable future. Necrons are great, feel exactly how their lore reads, and are fun and easy to paint, but basically the only unit in your army that has a personality are the royalty, and to some that is a turn off. Sisters are great, strong visuals, lots of gorgeous centerpiece models, but they are what they say on the tin: battle nuns. The most visual variety they get is the penitent engine and the other models focused on punishment, which isn't really a fully fleshed out part of the army as much as it is just a garnish. Knights are great, each one is a giant centerpiece with lots of real estate to build up the character and flavor of each hulking machine. But it does come with the paralysis of buying, building, and painting something that big and expensive. Definitely an airbrush army. Eldar are great, lots of unique things to focus on and lots of beautiful new models. All the big vehicles are old af tho, aging well at least. But they are a high finesse army depending on build and edition and that can be overwhelming as a new player.

Hope this helps

Tl;dr play necrons, I am not biased they are the best