http://hackslashinc.com/1/tpb-guide/
Omnibus 1 - Collects: Hack/Slash (Euthanized), Girls Gone Dead, Comic Book Carnage, Hack/Slash vs Evil Ernie, Land of Lost Toys 1-3, Trailers, “Slashing through the Snow”, Slice Hard Prequel, Slice Hard, and Hack/Slash vs. Chucky
Omnibus 2 - Collects: Hack/Slash 1-11 (DDP), Annual 1 featuring the Suicide Girls, portions of 12 and 13, and 14-17.
Omnibus 3 - Collects: Hack/Slash 18-32 (DDP), Entry Wound, Hackoween, and The Living Corpse Annual 1.
Omnibus 4 - Collects: Hack/Slash 1-11 (Image Comics), Annual #2: Murder Messiah, Annual #3: Hatchet/Slash, Hack/Slash meets Zombies vs. Cheerleaders, Hack/Slash 2011 Holiday Special (never before in print).
Omnibus 5 - Collects: Hack/Slash 12-25 (Image Comics) and Mercy Sparx: A Slice of Hell
Omnibus 1 is sort of a mix, even when I was reading the books as they came out I wasn't totally hooked. The Hack/Slash vs Chucky book kind of tied it back together and I started liking it more, then got really hooked with the DDP series (Omnibus 2-3).