Finally, magic banners. Keep in mind, a few Dark Elf units can take them as well as the BSB: Warriors, Corsairs, Witch Elves, Executioners, Cold One Knights, and Black Guard.
Banner of Nagarythe
It has a nice ‘fluffy’ feel to it, but it's so expensive and hard to imagine it’s going to be any more useful in 8th than it was in 7th. That's to say, don't expect to see it.
I've always liked the Hydra Banner, but it doesn't provide any help for supporting attacks, and with models being able to step up and continue fighting, you don't need 5 or so extra attacks in the first round of combat- you need more attacks throughout the length of the entire combat. Not useful.
You're going to be at the mercy of the charge rolls in 8th, but this is too many points to help fix that. In addition, there's not much benefit for charging except on Cold One Knights... but who is going to put a BSB in the Cold One Knight unit? There are better choices.
Having a banner in a unit that can cause either Fear or Terror may be nice, however, for the rare chance of it working (and the ability to ignore it from others), but I'd just go with the Fear banner Dark Elves have. I don’t see Fear/Terror being very potent in 8th, but doing it for cheap would be handy (as in one of the other banners).
Strider is a handy ability, and it's likely to save the skin of some of your models occasionally, but for the points you're better off being cautious about your movement. I will probably give this one a try at some point, however, because there is likely to be more terrain on the board, and I'll want a way to take advantage of my opponent's reluctance to enter some. We'll see just how much this is actually worth it.
Dark Elves get a much cheaper version of this banner in the Banner of Murder, but having a second unit with this ability could be very useful. You're able to give AP to multiple units of Black Guard, for instance. Armor Piercing is fantastic, especially for units like Black Guard. I'd really consider taking this.
Although I don't think Fear is going to be all that useful, it may be handy from time to time to be immune to it, or at the least, to give your opponent a chance to be WS1. I don’t see it happening or helping that much over the course of your games, however, so I don't know that I would bother spending the points on it.
Dark Elves (and most armies at the moment, I think) get this for only 25 points. I'm going to bet that you won't see this taken as often- your primary source of Combat Resolution is going to come from wounds, given both the amount of infantry on the table, as well as the amount of attacks you’re going to see from each side in a combat. I think you're going to be better off taking a banner that will either increase the amount of attacks you send out, or at least the quality of the attacks. Pass unless you really have need for more banners somewhere.
Standard of Slaughter
My thoughts on the above are still valid here, added with the fact that this is more expensive, and only useful if you get the charge. With charges being a bit more unreliable, this is probably not getting used again.
Standard of Hag Graef
This was a favorite banner for Dark Elf armies, especially for using on Black Guard since they could take it without buying a BSB. Since all combat is in Initiative order, it's hard to see much use. ASF + Higher Initiative gives you free rerolls for all rolls to hit... but your Black Guard are Initiative 6 and reroll all to hit rolls anyway, so you're only going to be taking it to deny ASF to other units (mainly High Elves). There's only a few more things your Black Guard are likely to face that this comes in handy- Wardancers, other Black Guard, Greater Daemons and Slaaneshi Daemons, and characters. Is it worth taking this banner solely as a counter to that? I'm thinking not- at least not on your Black Guard units.
If your BSB is taking it, there may be some point. There are a few options in this case. Executioners with ASF are actually pretty scary, but won't have enough attacks to be dangerous. A Death Hag BSB in a unit of Witch Elves is VERY scary, but will be very vulnerable to ranged attacks. Other than that, I'm not sure there's a role for the banner anymore. I don't like catering armies to handle specific things, and as this is a choice to handle such things, I'm not taking it.
Sea Serpent Standard
If you're taking a unit of Corsairs, you want to take this. Immunity to Psychology plus an extra attack really makes them a more effective unit.
Banner of Murder
This one will probably be in every army you take, somewhere. Even Spearmen with this are pretty dangerous. Armor Piercing is very nice ability, and with this you can get it on a unit for cheap.
Banner of Swiftness
Don't really see a need for this one. Dark Elf units are M5 or higher and going up to M6 really doesn't add all that much in the way of threat or utility.
I think you'll have an easier time getting MR from a Talisman, which can be taken by a few champions in a dark Elf army anyway. Otherwise, you'll have your characters able to provide the MR. Skip this.
Standard of Discipline
Not really much help in a Dark Elf army. Your Leadership is fine across the board, and Black Guard backed by a BSB aren't likely to break or fail any leadership tests. This is handier for other armies.
Banner of Cold Blood
This was useful for Cold One Knights to help combat Stupidity, but for now most of your combat units are likely to be Immune to Psychology, or at least have a high enough Leadership that your BSB reroll is going to keep them around. Another pass.
Banner of Eternal Flame
A Fantastic Banner. First, it causes fear in a lot of different things for cheap. Although I don’t think Fear is very useful overall, it's a nice side benefit. It negates Regeneration and deals extra damage to Flammable targets which is awesome. It also makes the unit AMAZING against buildings. It's also available at a very reasonable cost. I'd take it in every list.
Spend the points to get a BSB and ignore this. You're not going to have many units that are far away from your BSB with Dark Elves (given the units that can take banners). Any unit that would be off alone is probably well protected enough from Leadership tests, as well.
Fear, as I've said, is not going to be that big a deal these days, although it may be a nice ability from time to time. Causing Fear against a small subset of the models out there (even smaller than otherwise) is just not worth it. Take a better banner instead.