Best beaches SE Asia

Depends what you mean by "best". That can cover a multitude of features.
Best for water quality, sand quality, coves or swathes of beach, snorkelling opportunities, nightlife, vegetation, sporting activities, 24 h electricity etc etc ? You get the picture, you need to say what you personally like to get any useful answers.

Philippines probably.

Hoi An is ok, but nothing special.

Ngapali is expensive and also you will need to fly there from/to Yangon so factor in that cost. November it's best to book well ahead.

I second Herbie, Philippines has the most stunning beaches. Absolute tropical paradise.
Thailand also has them but Thailand is a mass tourism industry, really not much authentic there besides the beaches.
The other countries, Cambodia, Burma, Malaysia and Vietnam just don't have the same level of white sand + crystal clear water, if they do it's in hard to reach places.
Maybe Indonesia also has it, I haven't been there.

Vegetation! That's what I'm looking for, Friendly girl (who just may be that person).

Anyone with experience on Phu Quoc? Any downsides to Phu Quoc?

You can check the pictures and videos of Phu Quoc online. What do you expect? It looks like a nice beach in Vietnam. If you go to Philippines you have a ton of choices and everything is completely different because it's a different country, so I don't quite understand what you expect people to tell you.

I've been to Phu Quoc and I stayed in two locations.
The main beach is nothing special but has the advantage of being walking distance to town where there is a fabulous night food market with excellent (and cheap) seafood. The beaches get whiter and more deserted the further you go south and east and are easy to explore if you rent a motorbike. I stayed there at Cassia Cottage which was great, beach front and nice pool.
The beaches further up the West coast are nicer with better quality of sand and sea the further you get away from town but you need a motorbike to get around. Lots more nature there to explore from coves to pepper farms. Here I stayed at the Mango Bay Resort which was really nice. They have an enormous acerage of land for the few bungalows so you can really get back to nature here. Herds of cows regularily come by.

Phu Quoc has IMO the best beaches Vietnam has to offer but other SEA locations are better if it's purely beach you're after. Phu Quoc though is great for a "bit of everything" beach vacation with no major downsides.

I'd recommend having a look at the islands off the East coast of Western Malaysia ie. Perhentians, Redang, Kapas and Tioman. They are good all-rounders with lush vegetation, hikes, stunning beaches and good snorkelling.

Forget what I just said at #7. November will be very bad for weather.

You can check the pictures and videos of Phu Quoc online. What do you expect? It looks like a nice beach in Vietnam. If you go to Philippines you have a ton of choices and everything is completely different because it's a different country, so I don't quite understand what you expect people to tell you.

I was hoping that people would post about their recommendations and experiences on SE Asian beaches - in particular, beaches in countries covered in this (SEA mainland). Friendly Check-in Girl answered the question from her perspective, which was helpful..