Dark Elf core choices are overall pretty good, even in 8th, so you can't go too wrong with any.
Dark Elf Warriors: Good initiative and hatred, but not much else. However they are very inexpensive, so will likely be used as Sacrificial Dagger food for most Dark Elf armies. I would probably give them shields, even so, since they'll be slightly better at handling bow fire, but the other upgrades are likely not going to be used. Maybe a musician if you think you'll be moving, maybe a banner (for 6 points!) if you need it to have enough for Blood and Glory. You have the option of giving them a magic banner as well, but it's wasted on these guys. Take them if you want to run a Sacrificial Dagger, otherwise don't bother.
Dark Elf Repeater Crossbowmen: Same as last edition- lots of shots for a pretty good cost. 100 points nets you a 10-man unit of these, and since they can fire in two ranks, they aren't taking much space on your battle line. Not sure if I'd bother with shields most of the time, but if you need the points to get to the minimum core, it's a good way to spend them. Since you must have a certain amount of core, these guys make a pretty good choice, and they can do a really good job of causing some casualties on units as they advance towards you. As you'll need a certain amount of banners (to handle the Blood and Glory scenario) they may be a good place to put them as well (in this case, maybe give them shields). A musician may be handy since you don't want them running for long. I would probably skip out on a champion, however. A unit of 10 with a banner, a musician, and shields is a reasonable 125 points. Definitely take one or two units.
Black Ark Corsairs: I love corsairs. I remember putting them down during games against Sam (our Daemon player), and hearing him vocalize his hatred of them. They've always been very effective for me, and although they haven't really lost effectiveness, I think they've been overcome by units from the special slot (namely Black Guard). Their Slaver rule helps to run down units, but since other units are better for breaking opponents it's not likely to come up except as a supporting unit. Sadly, they just don't have much of a place anymore. Take Black Guard or Witch Elves instead.
Dark Riders: Still serve a similar, but reduced, role as in the previous edition. They are fast (and slightly faster because of their Vanguard move, as well as being allowed to take a Leadership test to march). They are still very maneuverable because they get free reforms. They no longer get the 360 degree line of sight for shooting, which is a minor loss, and now they aren't going to be breaking ranks since you'll need at least 10 to do so. I'll still end up taking a unit with crossbows and a musician, solely for harassment and occasionally hunting warmachines, fliers, or skirmishers. They are a great unit for going and getting behind my opponent for whatever purposes I need. They are still a fine choice, and for the same cost as crossbowmen, you get half the shots but a lot more mobility to them. In my case, they're also my only painted unit, so they'll have to be included ;).
Harpies: Our flying skirmisher unit, and formerly a staple of many Dark Elf armies. They don't count for our minimum core requirement, which hurts. I'm not sure they have much of a role anymore. March blocking is gone, so they aren't too handy for that (yeah, your opponent can fail a leadership test, but I don't consider that reliable enough a strategy to justify taking them). They aren't really any better for hunting down warmachines (Dark Riders and Shades can do it just as well). Skirmishers are a little weaker, since they actually have a facing as a unit. Shades are a bit more expensive, but they can do any role these guys can, and are more offensively potent. I don't see Harpies making many appearances in Dark Elf armies.
Assassins: Not really core, but they're listed in that section, so they get covered here. Assassins in the last edition were fantastic. They'd pop up, wipe out a rank of infantry, and would often turn the course of the combat all by their lonesome.
Now... that's not really going to work. Assassins are really going to be relegated to specific roles. I can think of two primary ones:
Character killer - Assassin w/ Extra Hand Weapon, Rune of Khaine, and Touch of Death (151 pts). You're looking at 5-7 attacks, always re-rolling to hit, and going first, and usually getting at least 1 killing blow result. That should do some damage. I'd also consider giving him Dark Venom to lose your Poisoned Attacks (since they can't KB) as well as REALLY take advantage of killing something in a challenge.
Scouting Assassin - Assassin w/ Manbane and Rending Stars (145 pts). Combined with a unit of shades, this will STILL provide a great amount of effective shooting.
I could see taking at least one, regardless. Probably the former, however.
So, with a 625 point minimum, what am I likely going with?
5 Dark Riders w/ musician, repeater crossbowmen - 117
10 Crossbowmen w/ standard bearer, musician - 115
10 Crossbowmen w/ standard bearer, musician - 115
10 Crossbowmen w/ musician - 105
25 Dark Elf Warriors w/ Shields - 175
Just enough. That gives us plenty of shooting, and plenty of bodies to power your spells!
Next up: Special!