Yes, EverQuest.
I have hundreds (thousands, who am I kidding) of hours in other games that I did enjoy, like Anarchy Online and WoW, but EQ was that first MMO experience I will never have again.
I remember so many wonderful and terrifying experiences from that game, but the one I remember most vividly was running on foot from Qeynos to Freeport for the first time at level 11? I had been playing about a week. I rolled an Erudite Wizard in Erudin, and leveled around there for a while solo-nuking snakes, and spiders, and other small things. When all of those turned green, I had to travel to Qeynos by ship. That's when I started joining small groups of other players in Blackburrow to fight gnolls. All of this was incredibly perilous by the way, dying meant that you lost XP, and you had to somehow run back to your corpse and retrieve it, or you would lose ALL of your gear. I have never seen a more severe death penalty in an MMO before or since.
Anyway, after things went green in BB, it was time to head to EC (East Commonlands). I had heard I could level up more in Oasis, and the central trading hub at the time (coordinated entirely through /ooc), was located at the EC tunnel. It was the place to be.
So I girded up my loins and prepared for the extremely dangerous trip. This was a cross-continental journey I had never taken before, never seen before.
I zoned out east from Qeynos Hills into West Karana, it was dusk which made for perfect coloration of that zone. Lots of auburn colors, cornfields, farmhouses, trees with yellow and orange leaves, etc. I was running easterly along the middle of the road, listening and watching very carefully for anything that I might accidently aggro and pull. I probably couldn't outrun whatever I pulled, and I certainly couldn't solo it, but it was all unknown back in those days. No guides, only the most basic maps, etc. So I'm moving down this dirt road, and I begin to crest a hilltop when I start to hear a distant 'scritching' sound. My footstep sounds stopped, as I decided to stop moving, and listen more carefully. It was getting louder. I looked around me and couldn't see anything, night was falling but there was sill some daylight, it was late dusk. Then to my absolute horror I saw a MOB approaching from over the hill, coming directly at me, indistinct, but HUGE. I was bigger than anything I had ever seen, bigger than anything I even imagined was in the game, and it was moving up the hill on the opposite side, in the middle of the road towards me...all I could see was a huge dark mass, like some kind of lurching unavoidable lovecraftian horror, pathing right for me. The 'scritching' got louder and louder, and I knew (from previous experience) that I couldn't outrun this. I moused over it and it was a RED MOB, it was SO MASSIVE, and I was still only seeing the top of it on the other side of the hill moving toward me. At that point I was literally frozen with fright in my chair.
I knew that I would die, I knew that I would lose about 4 hours work of grinding xp, I knew I would have to risk running back here again to retrieve my corpse before all of that or I might lose my +2CHA earring, and dagger, and robe that I had spent all of my copper and quest time grinding to find over the past week. But none of that registered until well after...all I felt in that moment was terror, and I KNEW I was dead.
The 'scritching' got UNBEARABLY LOUD (the volume was set very high so I could hear anything from a distance in order to try to avoid it, or run). I was waiting in abject horror, with my eyes closed, for the sound of that single attack swipe, and then the reload screen to announce my death, but it never came. Eventually the 'scritching' sound started to fade behind me, and I managed to open my eyes, and turn around. It was a Treant. I later learned that the reason it didn't attack me was because of faction rules. I was neutral to it.
None of the above would have felt real at all if it wasn't for the terribly punishing death penalty in EQ, but that experience was unlike anything I have ever felt in a cRPG before or since.