Who would win? Liliana Vess vs Iggwilv?

Battle in graveyard, so Liliana has the necromancy.

From a simulationist perspective, probably Iggwilv, but genre conventions give Liliana a pretty good shot. :)

Iggwilv is at least a Demigoddess in rank and is worshipped on multiple worlds (one of which is a fantastical version of Earth where she is known as the Finnish goddess Louhi)....so I'm gonna have to say The Witch-Queen of Perrenland....

To be fair Liliana did (spoiler for War of the Spark, the worst Magic story ever written)

 but she did so under very specific circumstances, with an army that she didn't create, and using the element of surprise. Not to mention, she would have died as a result if not for the timely sacrifice of an ally. So while there is some precedent for Liliana dealing with demigods, I still don't think she would win.

Iggwilv

I'm a few years late to this, but I feel that Liliana was unfairly and innacurately represented in the other comments, so I'm gonna hype her up a bit. Iggwilv really doesn't have any more reach than Liliana does. Yes, the witch queen is a demigod level being who is worshiped across multiple worlds...but so is Liliana. Liliana could have been worshiped more pre-mending, but she actively chose against it, once mentioning that several other planeswalkers create their own worlds and she simply "never got around to it." There is also a misunderstanding here in regards to her feats during the War of the Spark. For one, she did not kill Nicol Bolas, he is very much still alive. For another, the actual feat, which was controlling the God-Eternals, did not nearly kill her. Turning them against Bolas broke her pact with him, which he invoked to kill her. It had nothing to do with the strain of the feat. And what she did was certainly very impressive! The God-Eternals weren't just regular zombies, as eternals maintain all of the skills and abilities they had in life. Power-wise, they were still very much gods, and she was able to dominate and control multiple at once. Dealing with enormously powerful divine beings is absolutely something that is within her wheelhouse. Now, none of this is to detreact from the might of Iggwilv, who is quite impressive in her own right. I do think a fight between the two would be a very difficult encounter for them both, and I think it's a much closer match than other comments have presented. 

Quote from SnarkyGoblin >>

I'm a few years late to this, but I feel that Liliana was unfairly and innacurately represented in the other comments, so I'm gonna hype her up a bit. Iggwilv really doesn't have any more reach than Liliana does. Yes, the witch queen is a demigod level being who is worshiped across multiple worlds...but so is Liliana. Liliana could have been worshiped more pre-mending, but she actively chose against it, once mentioning that several other planeswalkers create their own worlds and she simply "never got around to it." There is also a misunderstanding here in regards to her feats during the War of the Spark. For one, she did not kill Nicol Bolas, he is very much still alive. For another, the actual feat, which was controlling the God-Eternals, did not nearly kill her. Turning them against Bolas broke her pact with him, which he invoked to kill her. It had nothing to do with the strain of the feat. And what she did was certainly very impressive! The God-Eternals weren't just regular zombies, as eternals maintain all of the skills and abilities they had in life. Power-wise, they were still very much gods, and she was able to dominate and control multiple at once. Dealing with enormously powerful divine beings is absolutely something that is within her wheelhouse. Now, none of this is to detreact from the might of Iggwilv, who is quite impressive in her own right. I do think a fight between the two would be a very difficult encounter for them both, and I think it's a much closer match than other comments have presented. 

The bit about her being able to be worshipped doesn't matter since, regardless of why she isn't being worshipped... she still isn't being worshipped. Which is a big deal in fights between divine beings. Iggwilv would have all that worship power to draw upon, and Liliana won't. Is that enough to turn the tide? Frnakly, I don't think Iggwilv even needs it to deal with her, so yeah, probably.

Quote from Lunasmeow >>

Quote from SnarkyGoblin >>

I'm a few years late to this, but I feel that Liliana was unfairly and innacurately represented in the other comments, so I'm gonna hype her up a bit. Iggwilv really doesn't have any more reach than Liliana does. Yes, the witch queen is a demigod level being who is worshiped across multiple worlds...but so is Liliana. Liliana could have been worshiped more pre-mending, but she actively chose against it, once mentioning that several other planeswalkers create their own worlds and she simply "never got around to it." There is also a misunderstanding here in regards to her feats during the War of the Spark. For one, she did not kill Nicol Bolas, he is very much still alive. For another, the actual feat, which was controlling the God-Eternals, did not nearly kill her. Turning them against Bolas broke her pact with him, which he invoked to kill her. It had nothing to do with the strain of the feat. And what she did was certainly very impressive! The God-Eternals weren't just regular zombies, as eternals maintain all of the skills and abilities they had in life. Power-wise, they were still very much gods, and she was able to dominate and control multiple at once. Dealing with enormously powerful divine beings is absolutely something that is within her wheelhouse. Now, none of this is to detreact from the might of Iggwilv, who is quite impressive in her own right. I do think a fight between the two would be a very difficult encounter for them both, and I think it's a much closer match than other comments have presented. 

The bit about her being able to be worshipped doesn't matter since, regardless of why she isn't being worshipped... she still isn't being worshipped. Which is a big deal in fights between divine beings. Iggwilv would have all that worship power to draw upon, and Liliana won't. Is that enough to turn the tide? Frnakly, I don't think Iggwilv even needs it to deal with her, so yeah, probably.

But they aren't divine beings. Iggwilv became an archfey, but her powers in that regard don't really extend beyond Prismeer, and a high-level planeswalker like Liliana, especially one bolstered by the chain veil, can stand up to high-level fey (plus, as per OP, the fight happens in a graveyard, which presumably isn't in Prismeer, since Wild Beyond the Witchlight never references any, especially given the wonky nature of death on the plane). The reason I referenced them being worshiped was to show that they had/have the power to be perceived as gods, not necessarily because they are gods (because, again, they aren't). Yes, Iggwilv has a son who is divine, but there's an argument to be made that that has more to do with being the son of Graz'zt, rather than the son of Iggwilv. Iggwilv could very well win, but I doubt drawing power from supplicants would be the deciding factor.

Iggwilv would win in my books, but I might be bias !

Iggwilv, but just because her abilities are a bit more varied (like more types of spells), vs. using zombies to brute force her enemies. Iggwilv has also fought with powerful beings and won on her own, and there is also the fact that while they both have immortality, which means lots of experience, Tasha just happens to be an archfey, which comes with the benefits of followers and a domain, while Liliana had to make pacts with demons, which actually made her lose her freedom, and eventually forced her to serve Nicol Bolas. However, I would agree that Liliana would stand a good chance against Iggwilv, as long as she had a sufficient supply of zombies. However, a pre-mending Liliana could have stood a signifigantly better chance against Iggwilv, and a pre Iggwilv/Zybilna version of Tasha (like the stat block presented in Vecna: Eve of Ruin), may be significantly weaker. Looking at them with dnd rules, however,  Tasha is presented as cr 19 in Eve of Ruin and cr 20 in Wild Beyond the Witchlight, while Liliana's character sheet (which was released on the MTG website as a pdf under MTG X DND Adventures in the Forgotten Realms) was only a level 10 warlock, which would give Iggwilv an easy win in either form. It is worth noting, however, that this character sheet was likely an after Mending interpretation,  but did include the Chain Veil in her inventory as her arcane focus, which didn't seem to alter her character sheet in any way.