Well, there are a lot of magic items in the new edition, so to save people from having to read one post with a wall of text, I'm going to break this up somewhat.
First, I'll start with weapons, and each category of magic weapons will follow.
One thing to keep in mind is that you have units that can take magic items: Black Guard and Cold One Knights. It's probably worth throwing a cheap item on them if you take either of those units, so that you can make them more effective (and give them a way to handle ethereal models).
Always Strikes Last is a huge deal when you're a T3 model on foot. Being able to do multiple wounds is nice, but even if you're using the ASF banner so that you can strike at I8, you're still not likely to get many swings. This weapon is of course even less useful given all the infantry you'll likely see. Ditch it.
Brings you up to S7, which is where you want to be to go after some of the big things in the game. A little costly, but it still leaves you enough points to take a magical armor (Armour of Eternal Servitude is a good choice) plus something small to give you some options. You can also take a mount, which can get you where you need to be.
Sword of Bloodshed
More attacks are going to be very handy in this edition. Combine with a Potion of Strength on your Dreadlord to really lay into big things. Of course, for 25 fewer points, you can take a Hydra Blade, which isn't reliable, but probably more cost effective. I likely won't be taking this.
Sword of Ruin/Obsidian Blade
I wouldn't bother with these, as you actually have fairly effective means of breaking through armor saves. Your KB assassins, Hydras, and Black Guard w/ AP attacks should do a fine enough job in that role.
Web of Shadows
Not worth it. 50 points to do a bunch of weak hits to a single model just isn't going to provide any benefit the army doesn't already have.
It's a fairly worthless item. It's only really helpful against characters (it's actually WORSE against single-wound units) or Monstrous anything, but it's not very likely to work against either. It's a lot of points for a slim chance to stop something from attacking.
Strength 6 is going to be very handy as well, but this is a pretty expensive way to go about it. For 15 points less, you can get the Crimson Death, which actually performs slightly better. Leave it out.
Sword of Strife
As good as the Hydra Blade for 5 points more. You'll probably take this over it, because it's more reliable, but if you need the 5 points for a combo or whatever, the Hydra Blade is a fine replacement.
This is mostly pointless, as you already re-roll to hit in the first round of combat. You're paying the points here for the chance to re-roll wounds. With S4, that's handy, but you're better off paying the points for more attacks usually, or more bodies in a unit.
This is cheap enough to stick on a hero with something else. It's great if you take a Potion of Strength, and it's random enough to be annoying at times, but overall it's a good value, and it's probably going to be very handy for a cheap hero when you're going against all those units.
It probably won’t to see much use. It's better than a lance for a lot more points. Negating Scaly Skin is nice against certain armies, and worthless against others. You also have to be mounted to use it. Perhaps with some of the Dragon Lord builds it may come in handy, but that will take a few games to determine its utility.
WS10 is interesting. Combined with the Ring of Darkness and Glittering Scales on your Dreadlord, you will make it very tough to hit you in close combat (as well as provide the unit with protection against shooting). They'll need to be WS 9 or 10 to hit your Dreadlord on anything but a 6. Of course, what role could he really fill? Who knows? I'd pass unless trying it out actually shows some merit.
Dagger of Hotek
Kind of pointless. If you're really concerned about getting ASF, you're better off taking the Banner of Hag Graef.
This is still pretty fantastic, especially now that the shots count as magical attacks. It's worth giving to your Sorceresses even. Put a Supreme Sorceress with this and the Guiding Eye (and maybe a Focus Familiar) in a unit of Crossbowmen (of decent size) and you'll have a fantastic shooting unit with a VERY impressive single turn of shooting at some point. Throw this in any list somewhere, just to take advantage of the easy magical shots.
Sword of Anti-Heroes
This will not be much use. Players are not going to be running multiple heroes in the same unit these days, and the benefits of meeting a single hero in unit are pretty light.
Still fantastic. Your Dreadlord or Master can take this to strengthen up their attacks and still have plenty of points for other magic items (combines well with many of them). You could also put this on a Champion in a unit of Black Guard for three attacks that always re-roll to hit. Another Magic Weapon that will get a lot of play.
Still pretty effective, as Killing Blow is still really helpful, but I don't know if there's a good way to get a character wielding this delivered effectively. I'll give it a shot from time to time, however.
Whip of Agony
An inferior Crimson Death, but this lets you take a shield to be only S5. Armor Piercing means you're still doing a good job on breaking armor though. If you've already given the Crimson Death to someone, this is a fine choice.
The effect of this sword will so rarely be useful; I can't see ever spending the points on it.
Sword of Swift Slaying
Again, for the most part, you're better off taking the Banner of Hag Graef instead of this if you're so concerned about getting ASF. I don't really see much point, since you're pretty high Initiative to start, so this is another item that won't get much play.
Really not a bad choice. Our Sword of Battle is the same cost, so with this you gain Immune to Psychology with only a small drawback. There's potential here, especially for various 'hard to kill' lords (since he can ignore a lot of Psych effects).
Not bad, but you'll need to combine it with something to provide either ASF (Banner of Hag Graef) or protection (Pendant of Khaeleth) to get the most out of it. It's affordable though, and it's probably worth it if you can afford ASF or the protection.
Sword of Striking
I'd skip it. You have hatred to help with hitting, and there are not too many cases where you'll really need the +1.
Sword of Battle
We have to pay 20 points for this instead of the 15 that's listed in the BRB. I'd take the Berserker Sword instead.
Sword of Might
Another item that's cheaper in the BRB. Only worth taking if you badly need a magic weapon, but even so it's probably not worth it. Most times, take a Halberd instead.
Gold Sigil Sword
Moving from I7 or I8 to I10 isn't going to get you going ahead of many things in the game. Skip it.
A fairly good item, but only really helpful against T6 or higher models. It's a cheap magic weapon, however, and you will need some magic weapons to handle ethereal models, so might be worth throwing on a character if you don’t have it covered otherwise.
Eh, Fear isn't as good as it used to be, so I don't think it's worth taking.
Cheap magic weapon, but Stupidity is not likely to provide much benefit, especially since you'll be in combat with the target anyway, which means they won't be testing for stupidity until either they've broken (and hopefully been run down), or you've broken (which is bad).
Don't bother. Better than the Tormentor Sword for cheap magic weapons, but I don't see it being useful enough to warrant inclusion. But only 5 points! If you need a magic weapon somewhere, this makes a good second choice (behind the Relic Sword).
Dark Elves have to pay 10 points for this instead of 5. Sad. If you need a cheap magic weapon, this also can come in handy... but you're probably better off taking a slightly more expensive option.