Welcome to the hobby newcomer!
Are we talking about
40K here? If you are then I'm assuming you're talking about Dark Vengeance; it's a great set to get started for small games with a friend and to test your painting skills, and can be expanded upon easily enough. However, Chaos Space Marines don't fare too well in current meta as far as I'm aware.
You shouldn't let that faze you though - buy whatever army appeals to you as you said, not what army is the best. However, if you're looking for armies that are currently in good stride, Tau, Eldar and Space Marines are in right now.
Hope this helps
Happy hunting!
G.A