DC has a Showcase line, which is pretty much their version of Marvel's Essential books, except in typical DC fashion, a bit more random in their release, content, and schedule. In fact DC's trade schedule and plan is far inferior to Marvels.
As for getting into Green Lantern, I suggest reading GL:rebirth and any GL Trades from Johns' run on the title. Older collections have some really cool stuff, but not really necessary since John's seems to be actively ignoring a lot of it and rewriting continuity on the fly (but that's a rant for another time).
Your milage on the various Crisis books will vary. Crisis on Infinite Earths is pretty much the very definition of Event, well researched and a huge epic scale. Infinite Crisis was ok, it tried to be as large and epic as the first and succeeds in some areas and fails in others. Final Crisis was Grant Morrison inhaling a bunch of drugs and writing a largely incomprehensible "story" of various scenes with little to no transition and relying on the fact that his fans will defend him by claiming anyone who dares criticize the story is just to dumb and philistine to get it and the true genius that it is. Known as "The Grant Morrison defense" you might be more familiar with it's modern name "The Inception defense."
Added at: 10:27
Sinestro Corps War. It's collected as trades which include both Green Lantern and Green Lantern Corps issues, so you'll have the entire story.
I second this. It was the best part of Johns run, certainly better than when he introduced the entire rainbow, and tried to insist and prove to us that half of the corps weren't just plot devices for blackest night.