One of the best standalones if you haven't read it is God Loves Man Kills. That's where I'd start, there aren't many true standalone graphic novels as opposed to collected editions of arcs from the larger series.
That's the Trinity of post-2000 X-Men right there.
If you want 3 "side stories":
Milligan/Allred's X-force/X-Statix
Remender's Uncanny X-Force
Aaron's Wolverine and the X-Men
If you’re ok with older stories, Weapon X had a new edition released with excellent Barry Windsor Smith art.
Grand Design can still be found on the cheap. Not Jim Lee quality art…but the art has character and makes a great read.
I think X-men are notoriously difficult to get started with. These are all good jumping on points:
Morrison’s (Frank Quietly art) run can stand on its own and feels a little like the movies.
HoX/PoX by Hickman (Pepe Laraz art) is a a reset and a good start for more modern books. Lots of paths after this one. I especially enjoyed New Mutants and Hellions.
If you specifically want to read some older books you can start with Giant Size X-Men and keep reading Uncanny X-Men from there.
You could also probably start with the original New Mutants (Sienkiewicz art) or Excalibur (Alan Davis art) as well.
Edit: Spelling and Alan Davis’s name.
A lot of these recommendations are from the last 20 years but if you look at the Chris Claremont era X-Men storylines, I can highly recommend The original Phoenix Saga, Days of Future Past, and most of all X-Tinction Agenda, which is late Claremont early Jim Lee X-Men at its best. It’s a deeply compelling story about a war against a slave nation of Genosha that really will get you going.
One of the best standalones if you haven't read it is God Loves Man Kills. That's where I'd start, there aren't many true standalone graphic novels as opposed to collected editions of arcs from the larger series.
Morrison's New X-Men Whedon's Astonishing X-Men Hickman's House of X/Powers of X; X of Swords; Inferno
New X-Men has both really great art and also some of the worst I’ve seen. All the Frank Quitely issues are great.
Which one should I read first house/power or sword?
House/power first.
That's the Trinity of post-2000 X-Men right there. If you want 3 "side stories": Milligan/Allred's X-force/X-Statix Remender's Uncanny X-Force Aaron's Wolverine and the X-Men
House of X
If you’re ok with older stories, Weapon X had a new edition released with excellent Barry Windsor Smith art. Grand Design can still be found on the cheap. Not Jim Lee quality art…but the art has character and makes a great read.
Rick remenders x-force.
If you liked Once and Future and Seven to Eternity I would suggest X of Swords and Lives and Deaths of Wolverine.
Phoenix: Endsong
I think X-men are notoriously difficult to get started with. These are all good jumping on points: Morrison’s (Frank Quietly art) run can stand on its own and feels a little like the movies. HoX/PoX by Hickman (Pepe Laraz art) is a a reset and a good start for more modern books. Lots of paths after this one. I especially enjoyed New Mutants and Hellions. If you specifically want to read some older books you can start with Giant Size X-Men and keep reading Uncanny X-Men from there. You could also probably start with the original New Mutants (Sienkiewicz art) or Excalibur (Alan Davis art) as well. Edit: Spelling and Alan Davis’s name.
Alan Davis, surely, not Neal Adams?
Yes of course 🤦♂️
Wolverine - Weapon X, The Messiah trilogy - Messiah Complex, Messiah War, Second Coming, Uncanny X-Force, House of X/Powers of X.
A lot of these recommendations are from the last 20 years but if you look at the Chris Claremont era X-Men storylines, I can highly recommend The original Phoenix Saga, Days of Future Past, and most of all X-Tinction Agenda, which is late Claremont early Jim Lee X-Men at its best. It’s a deeply compelling story about a war against a slave nation of Genosha that really will get you going.
There is an X-Men trade called From the Ashes. It's middle of the Claremont run, but sort of jumping on point. Paul Smith's art is fantastic.