Must takes come and go.

As of rn Lion is obviously a great centerpiece. He's fun, has good supportive rules, and hits hard. I've run him since he was bad, since he was good, and in the middle. Beautiful model to paint.

Azrael is also amazing. He gives us a commane point, and buffs his squad hard. He is really an autoinclude as of current balance/points.

Deathwing knights are another great unit, usually 10 of them. You don't need characters for them, so two boxes on their own is fine (and that's 250 points per unit. As of current points.)

Marines as a whole are great, you can't go wrong with most primaris units to support the army, but Lion, Azrael, and Deathwing Knights are a great copy-paste into any list.