Okay, sticking with just the stuff from the past few years and the trade paperbacks:
JLA: Tower of Babel is arguably the point where the situation that leads to the Crisis begins but I think Identity Crisis sets up the situation better and acts as a good primer to what is going on in the DC universe.
Adam Strange: Planet Heist is not absolutely required but it directly leads into one of the prequel Crisis series.
JLA: Crisis Of Conscience follows up on Identity Crisis and the last few pages (if I’ve remembered my issue numbers right ) lead directly into the beginning of Crisis.
Now for kind of a funny one. The OMAC Project TPB contains the Countdown special which makes it the place to start, but about halfway through it diverts into a different storyline that is collected in another TPB, Superman: Sacrifice, and then goes back to its own story. It can be followed without Sacrifice but if you’re going completely nerdy I’d recommend reading OMAC up to the point that Superman is talking to his father (you’ll know it when you get there), then reading Sacrifice, then switching back. One issue is collected in both volumes but the whole thing will make more sense this way.
The other prequel series can be read in any order. In the order of how much I liked them and how important they are:
Villains United has a pretty fun story that I would have liked reading regardless of the tie-in. Reading this before Crisis is good for the build up.
Day of Vengence isn’t a bad superhero story either, but the Crisis stuff seems rather tangental to me, especially in light of the reasons behind it.
Rann/Thanagar War really left me scratching my head as to why it was a prequel series. There’s a few pages that tie in but this is its own story.
Teen Titans/Outsiders: The Death and Return of Donna Troy collects a pair of miniseries and its the second one that matters for Crisis and goes back to the original Crisis as well.
Then there’s Infinite Crisis. There’s also some tie-in specials that I haven’t read yet since I mainly stick to TPB’s these days so I’m not sure if they fit into the middle or come afterward.
As for Crisis itself, it was a fanwank in the best possible sense. Light and fluffy with really nothing signifigant but if you’re a fan of DC comics then there’s something special for you in almost every part of it. Keep that in mind when you’re deciding how many of those you actually want to get.