Half-Life 2's upcoming 20 year anniversary on November 16th will be the worst.
Source: my ass.
The team that worked on the Half-Life 2: Update is working on Half-Life 2: Remastered Collection, so expect a similar celebration as with Half-Life 1. A new update that faithfully updates and improves upon the originals with a few extra touches on the side. Maybe even a Valve-made documentary about the creation of Half-Life 2, that would be nice. It'll still be more respectful than just about every other anniversary celebration nowadays.
I'm pretty sure Rockstar only released the GTA Trilogy Definitive Edition for GTA III's (and by extension, Vice City and soon to be San Andreas) 20th Anniversaries.
Edit: They might have released some T shirts in GTA Online too. Can't remember for sure.
2nd Edit: I think they also released the main trios default shirts and suit in GTA Online for the GTA V 10 year anniversary.
R*'s 10th anniversary celebrations are the best. They forcefully remove all the licensed music they didn't bother to expire the license for from digital copies. Fuck them, the GTA community showed more care for their titles than themselves. GTA V was a travesty and marked the death of R*.
R* didn't even acknowledge GTA IV's tenth anniversary. Only by
removing several songs from its radio stations, Vladivostok FM and Liberty Rock Radio being the most affected.
Yeah exactly that. I'm really mad over Vladivostok FM. It's thanks to GTA IV and this station I discovered Kino and Viktor Tsoi, but they've nuked about every song on that station aside from that one Seryoga song he made specifically for GTA IV.
To add insult to the injury, the new replacements they've added aren't period accurate culturally important picks like Kino, Leningrad, Splin, Glyukoza, Ruslana, or even Bi-2 that was meant to appear on the station with the song from Brat 2, which showed just how much old R* cared about showing appreciation and respect to their Eastern European audience, no. It's a bunch of random songs, a few from Seryoga, and then there's a song from 2010, 2011 and fucking 2016 thrown in there. Absolutely zero respect.
Those fan-made alternative stations are much more faithful. Besides the examples I brought up, they also have bands like DDT, Korol I Shut, Agata Kristi, Lyapis Trubetskoy, and they understand what type of music was meant to be on that station more than modern R*. But that would imply they even cared about keeping Vladivostok FM accurate in any way. They added those songs just because the license expiration removed so much that it would feel too out of place to only have one song on it.
LRR or any other station didn't suffer as much as Vladivostok FM. In case of LRR, the only songs that got removed were 1979, Edge of Seventeen, Evil Woman, Fascination and Heaven and Hell. Not enough to warrant finding replacements, but it was still bad. Well, any R* 10 year "anniversary" removal is bad, but what happened to Vladivostok FM was a crime against humanity.
Now besides sperging about GTA soundtracks, this'll go against the thread's theme because I don't know any other bad anniversary celebrations, but I do know a good one that's fairly recent. RWS' Postal 2 20th anniversary. The way they dealt with it was by releasing a fan-made patch for the game, xPatch, officially on Steam. It was made by a single Polish dude that's a die hard Postal 2 and old RWS fan, he has put a lot of care into it and ended up getting hired by RWS to do this update.
Besides fixing a shitton of bugs and rebalancing, he also backported a bunch of Paradise Lost features back into Postal 2, like giving NPC's the finger or dual wielding, and with all the old content restoration also came a new Classic Mode. If you don't know, the Steam version of Postal 2 is very different from the original game that released in 2003. A lot of assets have been updated and fan content has been implemented as a part of the base game. Classic Mode restores Postal 2 to what it was when it first released, since Chrzan is this much of a Postal 2 autist.
This is how you do proper anniversary celebrations, and Valve is doing the same for Half-Life 2 with the Remastered Collection. Let the passionate fans give your game justice, it saves you time having to come up with something and it shows that you care about the fans of your games, the people that made you who you are. But for that you need devs that give a shit about their legacy, and just about every major AAA company only gives a shit about the green line going up.