Frost (or another dragon, but almost certainly Frost).
Why?
First, the vision was not brought by the Power of Honor (a child), the historical Nohadon (Ishar confirms he was a petty king), or Dalinar himself (he was confused and shocked by the taste of Shin bread eaten correctly). These seem to be the other popular contenders, along with the entity in my next point.
Secondly, we can pretty safely discard any other entity (such as the "voice" of Adonalsium), since "Nohadon" references Midias/Wit/Hoid in real time. I don't find it plausible that the voice of the dead Uni-God is holding casual conversations with Wit.
Lastly, we know dragons can take on human form. We know they can communicate across vast distances and have Cosmere awareness. We know Frost and his sister are both in frequent communication with Wit, specifically in regards to the Contest and the intricacies of its contract and Alethi code.
So, my personal Frost theory is this:
Frost has been in frequent communication with Wit about Roshar, ROdium, Dalinar, etc. Since Wit and Frost don't always see eye-to-eye on a Cosmere level (IIRC), Frost initiated the first atypical vision from Oathbringer (also, heavy allusions to dragons abound in the vision). He did so in a form that would be familiar to and respected by Dalinar, hence the "Nohadon" façade. Later, when Frost felt Dalinar's Ascension, he reached out to Dalinar/Honor (Donor? Dalonor? I'm still workshopping it...) to offer a solution in the same façade.
So yeah, Frost.