1e. Hmm. Frankly, as much as the DMG has that nostalgia factor, it's a terrible mess of fiddly random rules (which some people like, but I do not). So I'm gonna abstain here.
2e. Monstrous Compendium was indeed pretty rad, and a welcome relief after those horrible three-ring-binder thingies.
3e. Manual of the Planes, hands down. This book is brimming with evocative planar locales, adventure seeds, and yes even some random tables. Every time I read this book it inspires me.
Honorable mention: Player's Handbook 2, for daring to change some of the core rules (around, e.g., druid's shapeshift) to make certain classes play better.
4e. Has the best DMG (and DMG2) of any edition to date, but I'm going to go with Monster Vault. As good as 2e's MC was, 4e's MV is better.