A good million of them to be frank, but let's try to keep it simple with just a few bullet points.
1) The first time I ever played, thanks to a free weekend on Steam. It was right after the "Demolitionist" update I think, and I remember playing Bedlam as it was the very first map on top of the list at the time. I remember how surreal and scary the map felt, I remember being totally trashed by zeds and not knowing how to reach the trader.
2) I cherish the friends I've made on the first Killing Floor. Most of them thanks to an old French Killing Floor forum that has been closed for a very long time now. I also made some in-game friends simply by playing on random servers too, although I lost touch with all of them now. I remember how all of us were so damn excited when Killing Floor 2 was announced! Sharing strategies and playing all night, always trying to get better, was also amazing, and I feel it helped me become a more strategic, but also relaxed gamer as a result.
3) I remember the first editions of each seasonal events, but also the very curious Portal 2 one... We were all trying to understand why Portal was suddenly linking to Killing Floor for some reason ! And coming back to the game with every new update, trying to grind the achievements, is another pleasure of mine.
4) I remember toying around in the first Killing Floor too. Exploiting the old bug allowing you to put pipe bombs on your head, acting as kamikazes for our team. I remember telling players NOT to put pipe bombs behind welded doors, as the Patriarch could simply detonate them all with a single rocket... only to watch half my team get wiped out for not listening. I just miss pipebombs in general man. I also remember the ludicrous weaponry you could find on the Steam Workshop. Most of them broken as hell of course.
5) I fondly remember a ton of great maps from that era as well. Some insane playthrough happened on Manor, Bioticslab, Office (SUPER TENSE), Wyre, Waterworks, Bedlam, Farm and Westlondon. But also some famous maps like Resident Evil 1 and Candlesmoke... real marathons testing your coordination and skills due to how long they were. Also some infamous ones, like the maps meant to level up your perks easily...
6) I remember when KF2 was first announced, and then all the promotional content we received constantly. From interviews to deep dive into the gore or melee system... It was wonderful ! It was also the very first game I beta tested, and each new update was massively promising and crazier than the last. As someone who was (and still is) playing mostly old games that aren't updated anymore, besides maybe thanks to the community when mods are a thing, it really felt exciting to simply... be there. To discover new stuff at the same time as everyone else, and sharing both the hype and theories with hundreds if not thousands of people. I really hope KF3 will bring that high back.
7) Simply getting lost in the possibilities. I've never been a hardcore gamer, and while I do prefer winning (like everyone I suppose...) and playing right, I do also like to goof around with whatever the game allows me to get away with. Running with both a Stoner and a Minigun? How about playing an entire game with just the 9mm? How about sticking to a theme : today I'll play as Ash Williams from Evil Dead, so I'll need a chainsaw and a boomstick... maybe it was born out of boredom, but I simply love doing random **** like that.
I'll definitely think about more specific examples later on, as those feels more like "core events" rather than memories. But nonetheless... Man, time flies. And I definitely had some of my best gaming experiences with the franchise. Mostly KF 2009 though.