Halo Wars 1 was a lot of fun. Simple and easy to get into for people who have never played or gotten into RTS games (like myself and a few others I know) at the time.
Halo Wars 2 adds more units, which opens up a lot more options for your strategies but just enough so it's not overwhelming. Generally I'd say I prefer 2 now due to this but it can take a little bit of getting used to.
Both are great games and I'd recommend them even to people who only like the FPS Halo's. If you're an RTS fan already, don't go into it expecting a full blown, in-depth RTS experience and you'll be fine.