Volume 1: Lobdell/Bachalo Part 1

Phalanx Covenant:
Uncanny 316
X-Men 36
Uncanny 317
X-Men 37

(Be sure to grab the non-Deluxe versions of the Phalanx issues, so as to avoid the heavy cardstock covers, and the inner page paper is exactly the same in both.)

Uncanny 318 (“Moving Day”) (Generation X prelude)
Generation X 1–4
Generation Next 1–4 (Age of Apocalypse) (I say include it: Bachalo’s art throughout is just too good to skip, and it’s a good story.)
Generation X 5–9
Generation X Annual ’95 (Mondo joins the school)
Generation X 10–11
Generation X Collector’s Preview

Volume 2: Lobdell/Bachalo Part 2

Generation X: Overstreet Fan Edition (easily found in the X-Men: Rarities TPB)
Generation X 12–21
Generation X Annual ’96
Generation X 22–28
Generation X -1
Generation X 29–31

Volume 3: Hama/Dodson

Generation X Annual ’97
Generation X 32–41
Generation X ½
Generation X 42–47
Generation X Annual ’98
Generation X Holiday Special

Volume 4: Faerber/Dodson

Generation X 48–50
X-Man 50
Generation X 51–58
Generation X Annual ’99
Generation X 59–62
(this volume contains 4 double-sized issues so while the map looks small, it might be the thickest by a nose)

Volume 5: Ellis/Wood/Pugh

Generation X 63–75
Generation X Underground Special
X-Men Unlimited 30 (“Banshee’s Angels” — Underground Special sequel, and in full color!)
Gen 13/Generation X
Generation X/Gen 13 (sequel to the prior)
Generation Hex (Amalgam)

For completists, one could always add X-Force 42 after Generation X #3, and Wolverine 94 after #9, but I didn’t think they really added much. There’s also Marvel Team-Up #1 (1997) “Starring Generation X!”, but I thought it was only OK. I would put it right at the beginning of Vol. 3 if I had to, though.

FYI, if you’re one of those adverse to unnecessary annuals, the only ones that need to be included are Annuals ’95 and ’99. They’re both written by the then current writer, and are major parts of their storylines.