Day 25 is a special day for me, so I am really hoping there is a special question.
Well alright. Not bad at all.
Let's give this some thought. Hmmm. OK, I've got it...
In my opinion, the best way to thank your GM is to invest in, and pay attention to the game they are running.
What I mean is that if your Gamemaster is putting work into creating distinctive and interesting NPCs, interact with them. Don't automatically assume they are untrustworthy, or unimportant. Get to know them.
If the GM puts time and effort into developing an interesting locale for a scene, or a battle, make use of your PCs surroundings, and embrace being hindered by the slippery rocks beneath a waterfall, or the exhausting heat of a desert.
Thank the GM who cares about their craft by treating the campaign world as more than just the background of a video game. Be a part of it. Get to know how it works. Pay attention. Immerse.
Over the last few years, I've had a number of players do things to 'help me GM'. This 'help' came in the form of creating details that didn't match what had already been established, and getting the party together in a location through an derived method that made no sense considering where we left off the previous session.
Time, and again I was told that they [the players in question] were just trying to help. They were creating content so I didn't have to. They were getting the group together to make the adventure easier to manage.
I can't tell you how much that annoyed me, and how unhelpful it was.
It told me, "We don't care about, or weren't really paying attention to the significance of the events you worked so hard to establish, and coordinate". It said they'd rather do their own thing without thought to the GM, the setting, or the other players.
If you want to know the best way to thank your GM, it's by enjoying and appreciating the work they've put into the fun you've had, and keep having.
Don't avoid the awesome villain in the Superhero RPG that your GM took the time to create. Confront it, as a superhero would. Don't spend an hour, or more of real time scanning the mystery at a distance in Star Trek, hoping to avoid any potential mishaps that might arise. Scan once, or twice, take the results, make a plan, and beam down.
Your GM made an adventure for you. One that a character of your type and personality would want to be part of. So, go on the damn adventure!
You can thank the GM by not wasting his, or her time, and the time of your fellow players, You can do it by actually participating, coming to a gaming session, with the intent to game.
After all, that's why they, why I, spent a week, or two working on the game. It's so it would be played.