dgraham414 said:

At this point, I don’t at all think Halbrand is Sauron.

Eeeeeh I don’t know about that!

I have a list of clues:

Halbrand’s very first line is “The tides of fate are flowing. Yours may be flowing in, or out.” This is something Galadriel says to Frodo when she is tested by the ring in Fellowship. Basically, her deciding NOT to become Sauron.
30 seconds later, he says “Looks can be deceiving." Very suspicious opening statements by someone who is NOT Sauron the deceiver.
In contrast with the other “unwilling king”, Aragorn, Halbrand has a streak of cruelty, from pushing off his companions on the driftwood to get eaten by the worm to brutally beating up and breaking the bones of the Numenorians in the street fight (even after they were done with him and walking away).
When adrift at sea, Galadriel calls attention to his medallion, of which Halbrand mentions he “found it on a dead man”. I feel like this is the kind of line they put there to show him being funny or chummy, but really we need to consider that it might be the literal truth.
Halbrand’s speech about giving people a “means of mastering their fears so you can master them” feels like a good strategy for corrupting people. I don’t think a true King in Tolkien’s world would subscribe to this type of manipulation.
When Galadriel tells Halbrand about the mutiny of elves she says “they could not distinguish me from the evil I was fighting”. Half a second later, rack focus to Halbrand.
In the same scene, Halbrand says he’s sorry about her brother. It seems like an actual apology. I think what may be happening here is that Sauron may have sincerely tried to start over after the defeat of Morgoth, and since he has the ability to change appearance he intended to disappear in Numenor.
Halbrand is very good at making weapons. I remember from Lord of the Rings that “the hands of a king are the hands of a healer.” We see Aragorn try to heal Frodo with the kingsfoil after he’s stabbed on weathertop, and later he heals Eowyn after the battle. Again, in Tolkien’s world I don’t think a true king’s greatest skills would be smithing and manipulation. That sounds more like someone else.
Adar says he killed Sauron after having sacrificed so many orcs for his experiments. Halbrand seems to remember Adar, but not the other way around. Perhaps Halbrand was previously in another form.
Possibly the biggest clue yet, as Halbrand is walking out of the room, Adar asks “Who are you?” Cut to Halbrand from behind, who pauses slightly, doesn’t answer and then leaves. I feel like this moment wouldn’t exist if we are to take Halbrand’s identity at face value. So what identity would make for a satisfying reveal? My backup idea was the Witch-king, but I don’t think the Witch-king is someone that is revealed rather it is what someone becomes.
After the battle, when Halbrand is talking about the feeling he got fighting alongside Galadriel, he says he wishes he could “bind it to my very being.” That word bind reminds us of “one ring to bring them all, and in the darkness bind them.” It’s just too suspicious to be coincidence.
I’m not a makeup expert but I feel like they’ve given Halbrand some heavy eye shadow in such a way that to me he almost looks like an evil-Aragorn. Just waiting for the goatee reveal.

Finally, and perhaps most importantly, I think if Halbrand is Sauron it would be the most compelling development for Galadriel’s character. It plays into her brothers words, “Sometimes we cannot know [which light to follow] until we have touched the darkness.”

Galadriel has been headstrong and determined all season. She’s also been stubborn and arrogant, and I think she’s in for a rude awakening. She basically dragged Halbrand back to Middle-Earth, gave him an army, and crowned him king, all because of a medallion that he “found on a dead man”. How crazy would it be if Gil-Galad was right that “the same wind that seeks to blow out a fire may also cause its spread”?