The title certainly can relate to multiple characters, no doubt about that. “Truest” sounds specific to one person though.
GOAT are the truest believers we’ve seen thus far on the side of the badguys. We already know how Greg justifies his beliefs and commitment to the cause, it’s Tamara who is the blind true believer judging on what we’ve seen so far.
Henry is the truest believer we’ve seen thus far on the side of the goodguys. Even when he was the only person in SB who believed, even when Regina tried to get a shrink to talk him out of it, and when Emma kept telling him she didn’t believe it too, Henry still stuck fast to being a true believer.
And Henry is with Tamara, so you’ve got the two main “true believers” stuck with each other at this point in time. That’s what’s making me think they’re the specific ones the title refers to, even though it can really apply to multiple characters in different ways, and we’ll likely see in this ep how each of the other characters believe so strongly in their own views.
PS. Four pages so far trying to dissect a simple episode title. We are nuts. I heart the spoiler forum! LOL 🙂