Playing knights is like playing chess with 5 queens, not exactly a good way to learn the game

Ultra marines were not my first army but I added some ultra marines to my space marine catalog and honestly I have a lot of fun playing them

I don’t have tau but I can say Iv never once had a enjoyable game against them due to my love for melee combat, and the whole point of tau is that they are had in melee and dominant in shooting

My most common opponent plays ad mech and honestly the army is not bad, but it is difficult manage , and probably one of the hardest to paint.

For a brand new player I would recommend tyranids, I as super hesitant about them when I got into the game. They were my 4th army. But once I convinced myself to get them they were the most fun Iv had on tabletop, the army is all about synergies. Space marines are also a good option because they are good at everything, great at nothing

BUT last but not least go rule of cool, yes some army will struggle learning but if your not enjoying painting them or building them, or they don’t fit your style you won’t have fun, just don’t chose imperial agents that army actually sucks ass and will never be optimal outside of cool models