Its hard to answer this question with 4e because it really is just filled with awesome stuff. There was never a book you would say "don't bother with this."
That being said... What do you NEED to play? You really should have DMG1-2 and PHB1-2. You can go with MM3, MV, and MV:TtNV, but there ARE some holes in that assortment of monsters. WotC never bothered to really reissue a full set of dragons for instance. While there are a number of demons and devils and other 'classic' monsters in the later books, a lot of the standbys are in MM1-2. Still, you can always get the later books first and the earlier ones later if you feel the need.
There are a LOT of other good books. The Rules Compendium is a nice thing to have, it has a lot of cleaned up core rules, the best version of the SC rules, etc. Anyway its a cheap book. Beyond that ALL the GM facing books are interesting. Some are really not needed unless you want to focus on what they cover (IE if you want to do adventures in the Astral Sea, then The Plane Above is a good book, otherwise its kinda overkill). The Open Grave was about the earliest one, that's kinda dated, but STILL has a lot of good stuff if you want to dig into undead. The Draconomicons are a little old too, the monsters in them could use a refresh, but AGAIN they are filled with great material. Underdark is cool, etc. MotP is very handy if you plan to do anything at all in the planes.
The player-side books are all quite nice. Some are even close to mandatory for some classes. Arcane Power for Wizards and Warlocks, Divine Power for STR Paladins (paladins in general really), etc. Its hard to dis any book. AV1 and AV2 MIGHT be considered obsolescent due to MME, but they STILL have a lot of stuff in them that is considered pretty vital for most builds. Post-Essentials books are even more cool in that the production quality gets better the later you go in 4e. Heroes of Shadow, Heroes of the Feywild (the best D&D supplement EVER IMHO), and Heroes of the Elemental Chaos are all pretty cool.
The only stuff I might skip is the DSG, which was the last 4e book to come out. I kinda felt like it wasn't trying that hard. It has some decent stuff in it, but nothing really crazy good. The BoVD came out around the same time, and I found that one more interesting, though again its not like you NEED it.
PHB3 is sort of an oddball. Frankly I never crack it open these days. If you want to play with Hybrids then its useful. Otherwise there's not too much in there that you NEED. The classes are forgettable, the Monk excepted, and the races are highly niche (I liked Wilden, but they certainly aren't something you NEED).
