Heroes of the Feywild is a definite must and is probably the crown jewel of the player side Essentials line (I'm particularly fond of the Skald with its extremely versatile ability score options; Str, Dex or Cha could easily be primary scores since its effect trigger off of basic attacks).
Monster Vault: Threats to the Nentir Vale though wins best Essentials product hands down. The monsters are bar-none some of the best designed in the entire game and have a rich backstory to them that makes it very easy to come up with adventures to actually use them. The only real downside to it is that most of the foes in it are heroic to low paragon tier in level (you could always level them up yourself of course).
I'm also fan of having both HotFL and HotFK classes (the Scout from the latter is one of my favorite striker classes, even before the errata to fix power attack so both it and its offhand attack could be used in the same turn) and we've used pretty much all of them at some point or another in our home games (it may not be 'top tier' but it definitely wins 'simplest controller' hands down... making it a favorite for our use in smaller home games where we have 2-3 players running two PC's each so we can have all the roles covered and have a wider array of monsters to fight).
Mordenkinen's Magnificent Emporium is hard to find in print, but is to the Essentials line what the Adventurers Vaults were to the original line and definitely worth it. As with all the Essentials line, there's a lot more fluff associated with each item as well.
Heroes of the Elemental Chaos is decent, though the Elementalist as an actual simple arcane class is the biggest draw in that regard. You could probably live without it.
Heroes of Shadow is easily the most skippable of the Essentials line.
If you want some of the original classes (like the Warlord) back in the mix, pick up the free PDF entries that were going to be a part of the Class Compendium... they're essentially the PHB1 classes with the errata applied to them and some feats that help you integrate options from the various subclasses into each other (i.e. a fighter with power strike or a knight with an actual fighter encounter attack power).
ETA: As a shameless plug, grab my Essentials Options article (found
HERE) for some additional fun options for the Essentials classes (along with daily-less options for pre-Essentials classes).
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