December destination opinions please!

I'd go to the one that interests you most. What else do you expect anyone to tell you? If I knew you well as a person I might have an opinion as to which you would enjoy more but as a stranger I have no way of making that call. Which I would prefer is of no use to you unless you knew me well as a person and had some basis on which to consider how much my opinion might be like or unlike your own.

Of your 4, I personally would choose Ethiopia or Georgia/Armenia. But what does that tell you? Nothing. If I was a big fan of one of the four, I could wax enthusiastic about it and try to 'sell' it to you. But again, what value would it have to you?

So again, my opinion is YOU should decide which interests YOU the most.

Georgia Armenia is going to be really cold at that time. Better to save your visit when the weather is warmer.

Philipines is in Monsoon season November and early December. Again, maybe better to visit aother time of year.

So that leaves Thailand/Angkor Wat or Ethiopia (weather is great in both places November and December). Based on your previous travel highlights Ethiopia sounds like a better option. There is certainly trekking options there (Simien and Bale Mountains), however I'm assuming no scuba diving as it has no coast. Obviously Thailand has a lot of great scuba diving so if that is a big thing for you maybe you should consider Thailand.

scuba seasons for december has a nice map view of most dive sites for this time of year around the world. :)