Basically all beaches in Thailand are beautiful
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I love Thailand's beaches. I think they are some of the most beautiful in the world and unlike some of their contemporaries on other places in the world, they aren't super expensive to be able to hang out at. This is part of the reason why the beach areas in Thailand are so popular globally.
Think about it ok? Where else can you go in the world and have perfect white sand and emerald seas in front of you?
There is one thing that is not so great about these beaches though and that is that apparently, they are really dangerous to the point where we hear stories about people drowning in them.

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The above is a fairly good example of just a run of the mill beach in Thailand although whoever took this photo likely had to wait a while or photoshop out dozens of people that were there.
I recall the first time I encountered a beach of any sort here. I was on a motorbike and I had to pull over just to marvel at the sight of the majesty that was in front of me. As time went by I would later figure out that this beach, which initially blew my mind, was one of the lessor beautiful ones and wasn't and isn't very highly regarded.

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I can't find the pictures that I took but this is the same beach called Hat Nopparat Thara and it is a long beach of wonderful sand and well, even to this day almost nobody uses it. Why, well that is because there are so many amazing beaches in Thailand that ones like this aren't considered special.
The good news about Nopparat Thara though is that it is very easy to get a parking spot and this is generally not the case at some of the more famous beautiful beaches.

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Patong Beach, for example, is considered one of the most beautiful capes in the entire country but unfortunately, due to its fame, the above picture is what you are likely to encounter there all the year round.
There are others that are just as nice and not filled with people 24/7 but in order to get to them you are going to have to work a bit to get to them and it may take a little while to reach it. This was the case recently with my trip that I took to Raylay Beach. Arriving at the rather minor Krabi airport and then taking a taxi to the pier and THEN taking a tiny boat to a pedestrian only peninsula AND THEN walking to your bungalow operation...well, a lot of people aren't willing to go through that.

There are still going to be other people around. It is unavoidable. But given the amount of work that you have to do in order to get to these places ensures that a lot of the masses that will go to a place like Phuket, are not going to go to a place like this.
Another island that people who want to have the place to themselves a bit more like to go to is Koh Lanta. In order to get there you have to get a flight to the rather small Krabi airport and then suffer through about an hour or more in a taxi to get to your bungalows. But once you get there, provided you left enough time, you aren't going to want to leave. You also are not going to be pestered constantly by sales reps and you aren't going to be in a massive crowd everywhere that you go.

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The above beach called "Long Beach" in Koh Lanta is named that because it is a long beach and I love the simplicity of that. This island gets its fair share of tourists as well but because it is so far from an international airport, the people who have limited time don't consider it as a destination. This is part of what makes it to wonderful.
There is still plenty of development on the island so it isn't like you are going to struggle to find something to eat or anything. There are likely hundreds of restaurants. It is also a rather large land mass that I think is about the same size as Phuket but while Phuket is completely overcrowded, Lanta feels almost like a small village in comparison.
The prices are also much lower in a place like Lanta.
So what are the downsides? To me there aren't any other than that fact that it isn't very feasible to go to Lanta if you only have say, the weekend off and are traveling from very far away. In that regard I guess Phuket is more convenient although for me, Phuket isn't convenient for anything because of the overcrowding.
The only downside that I can think of as far as Lanta is concerned is that its rather remote nature makes it so that having just any food that you can imagine available isn't possible. That's kind of ok though because as you may already know, Thai food is one of the best cuisines in the world and of course they know how to make that in Lanta.
You can't really go wrong at beaches in Thailand though and I have been to some rather remote islands such as Koh Phayam that doesn't have any roads for cars and up until recently didn't even have electricity all day as this was provided exclusively by diesel generators. They have since installed mainland electric and I bet that kind of makes the place feel a bit more modern.
The bottom line is that you can't really go wrong when selecting a beach in Thailand because they are all pretty darn awesome. For me, I find the ones that are less well-known and less traveled to be the better ones simply because the popular ones such as Samui, Pattaya, and Phuket are so overrun with tourists that it feels like I am waiting at a rather beautiful train station in a metro area. I don't like to feel like that on vacation and I don't think most other people do either.
So if you are considering traveling to Thailand for the beach life, I think you would be doing yourself a huge favor if you simply skipped the major ones, the well-known ones, and instead intentionally sought out something that is going to take hours to arrive at.
I believe the payoff is well worth the extra time it takes to travel to it. You will never be in the middle of nowhere at these beach areas and I have been to a lot of them. Even the more remote ones like Koh Phayam I ended up meeting a bunch of people pretty rapidly because we felt a bond since we had all took the extra time to travel to somewhere that isn't just the same thing that everyone else is doing.
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