Voidravens can be pretty potent on the table from what I've heard; there's a reason many people dread playing against them, so if you're going for effectiveness that's a good one. A nice looking model too!
The Talos is a fantastic model in my opinion, though I have a soft spot for the adorable creations of the Haemonculi. I would consider it well worth the ~46 dollar cost for the model alone, let alone the fact that you can use it on the table as well (the beauty of wargaming!)
The Tantalus I don't have much experience with seeing or hearing of it's effectiveness, but again it's a very nice model that would make a great centerpiece for your army. While 100 Euros seems fairly steep, again it's a sizable and detailed model, and of course made of resin, which always drives up the price. If you've been eyeing this one for a while now might be a good time to purchase, because as you said it's the most expensive of the three choices, and a time like now when you have some extra funds lying around is good opportunity. Sorry if I didn't give you as much advice on effectiveness as you may have wanted, I'm fairly new and I don't know a lot about how specific models are other than general knowledge relayed to me by others. Whichever you decide with is a good choice in my opinion. I'd encourage you to post a picture to the painting forum when you're done with them, I'd love to see it!