That happened before, remember? It didn't go too well for the Elves, but in the end they stomped the proverbial sh!t out of the Chaos hordes.
Really though, if it weren't for a heroic last stand to protect a coven of the best mages to ever grace the Warhammer World (quite possibly able to quell the winds of magic from even Lord Kroak), then Chaos would have won that stand, and a war of attrition would have certainly wiped every last sentient organism from the world at that point.
With that, it's obvious I'm a big fan of High Elves. They're the defenders of the great Vortex, the ones who allow all the other races to walk without wading through pure Chaos. They're a dire necessity to the world (not so much for Chaos, though).
Dark Elves are corrupted and disengaged from their elven civility, though they stand in stark balance to the High Elves. The Dark Elves also serve to mitigate the Chaos tides that wish to travel southward by land. With ease, mind you. Hence they actively limit the Chaos forces, while acting as a balancing party for others abroad. Dark Elf raids are quite fearsome, from what I gather.
Wood Elves, on the other hand, only fend off the Beastmen from invading their own forests. They rarely lend aid in any form to any other faction. They play a strategic defense for their own purposes. Should the rest of the world be enveloped by Chaos, and the woods of Athel Loren go unnoticed by the vast hordes, they won't care.
Hence, I argue that the Wood Elves have the least importance in the Warhammer World, so I vote they are the "worst kind of Elves." Don't think I'm saying I despise the Wood Elves. Rather, I respect the idea of Woodland Elves. They're a splinter faction of the High Elves, after all, and I like the idea that the Wood and High Elves could pose a stand with the humans of the Old World should the need arise (and the Wood Elves be pulled into the affairs of others...).