>>718595085
In order of recommendation
>Dawn of War 1 + expansions
One of the best RTS games of all time, with tons of content, a solid learning curve, great voice-acting / OST, visuals that hold up well and fast paced gameplay
>W40K: Space Marine
It's a fun third person shooter with an emphasis on melee combat. Unfortunately the game's MP is long dead, but the campaign is still enjoyable outside of the final boss
>W40K: Mechanicus
A decent turn based strategy game that is very similar to the modern XCOM games. The difficulty curve is kind of fucked because it gets considerably easier as the game goes on, due to your handful of Tech Priests becoming absolutely OP so long as they don't die. Good presentation and faithfulness to the lore carry it, although the combat is still solid.
>Dawn of War 2 + expansions
Not as good as the first game, but DoW 2 has two enjoyable campaigns that are much smaller scale with more of a focus on individual squads using different equipment and abilities. The skirmish mode is kind of ass, but the Last Stand mode is a lot of fun and still has a small group of players
>W40K: Boltgun
Boltgun is a solid W40K boomer shooter, although the first half of the game drags on a bit in my opinion. Once it gets going it's a fun FPS experience with nice visuals and fast paced combat. Again it has a decent presentation and lore faithfulness

Most of the rest I can't really recommend in good faith (Rites of War, Kill Team, Inquisitor Martyr, Deathwatch, Space Wolf, Sanctus Reach, Space Hulk etc) or I haven't played yet. Legacy of Dorn: Herald of Oblivion is a nice adaptation of a gamebook if you can find it - although it was always niche and got delisted years ago. It was also on Android so might be easier to find the .apk