I've always been a big fan of Eldrad and indeed he's the only model I kept when my Eldar army went on eBay. The issue for some is that he doesn't fit in with jetbike seer councils but there's no arguing he's a powerful psyker. The combination of the Runes of Fate powers with Divination and Telepathy is pretty fearsome. Obviously, it's open to randomness but even with the Primaris powers of those disciplines he'd be potent. Whether or not we'll see him as much remains to be seen as I'm not sure farseers are necessarily as automatic a choice as they used to be but more on that later.
He'll likely need a transport to be effective and without an assault vehicle he'll have to take some punishment on the way in. He's a decent choice but I suspect most will go with a standard autarch if they want reserves manipulation. They're pretty good for the points (but I'll get to them) and Yriel doesn't really offer much that they don't.
Elsewhere his warlord trait gives him split fire making his unit more potent. His weapon is 48" in range and has the Distort rule making it great against characters and decent against vehicles too. His Hidden Path rule is interesting allowing him to Infiltrate wherever he likes and allowing other rangers to deep strike to him. Interesting but difficult to see it's effectiveness. For normal rangers I can see the benefit as it allows them to get in behind units and pick of models that are staying to the back for safety. For pathfinders it seems a bit pointless. Perhaps I'm missing something but I'm not sure I really want him that close to my enemy in the first place so I'd be more likely to keep him outside 18" anyway.
Most will see him as a "tax" worth paying to get Pathfinders. Personally I think it makes them too expensive for something that's still vulnerable to flamers etc. Rangers as a cheap scoring unit are probably still a viable option though but again I'll be getting to them.
The Avatar of Khaine
He's now also eligible for a couple of upgrades. Realistically he's only going to use Fast Shot mind you. A two-shot BS10 meltagun is tempting. Should I ever get a full army again I think it'd be fun to run both an avatar and wraithknight! Once either of them is in combat they're likely to be pretty unstoppable.
None of the above are bad by any stretch but in a codex with a wealth of options they perhaps all could do with being a little better to justify their expense. Certainly Yriel and Illic are easier to justify as they're cheaper but generally they'll be a play-style choice and perhaps unlikely to make competitive lists.
Personally I'd love to try any and all of them in my lists but I need to think carefully before splashing out on the models.