I must apologize for this post ahead of time; I know it will be long, rambly and full of uninteresting nostalgia -- sorry!
At first glance, this thread simply read as a 'who is the most skilled player you've come across', but I ended up interpreting the word 'best' in a plethora of different, and to me at least, unexpected ways;
The best memories I have of playing Everquest do not revolve around epic quests, server first raid kills, getting that BiS piece of gear, but instead rather my very first guild, I was a completely clueless barbarian shaman, and I was invited to join a barb only guild on Cazic, my first server. It was full of helpful and kind people that patiently endured the endless barrage of incoherent questions coming from my 12-year-old brain. For the life of me I cannot remember what the guild was called, but I loved every second of it.
The best player I have ever played with I met a few months later, he convinced me to make a new character on his server (The Nameless) so that we could play together. He played a wizard called Arbaal. It was through him that I learned how to actually play the game, he also helped me understand the concept of raiding, of raid guilds of playing mmorpgs in a more competitive way. All of which in turn made me a better player, not just in Everquest, but in every game I have played since. From then on we have played a bunch of games together, from both versions of Starcraft to WoW, ESO and even Half-Life -- and he was incredibly proficient in all of them.
The best guild/raiding experience was probably beating Emperor Ssra with Remedy and finally getting into Vex Thal. The absolute slog to gear up, farming bane weapons, shards and doing the key quests made the actual kill feel like an actual accomplishment. I also really enjoyed the zone layout and all the snakes! A perhaps petty but truthful part of me wants to give an honorable mention to our biggest achievement in PoP, which was cockblocking Aerin'Dar from Legacy of Steel for what felt like an eternity, the flame wars and forum drama was fantastic!
The best boxer/farmer/bazaar person while hotly contested must go to someone I consider a close friend (but shall remain nameless). He spent so very many hours farming Sol Ro's tower that he had complete control over the ornate armor market, he was great and efficient at multiboxing (without software). I was just always very impressed!
Lastly, the best person I ever had the pleasure of meeting was a cleric that was just universally loved by everyone that ever encountered him. He never had anything bad to say about anyone, and no matter how busy he was, he always had time to lend an ear if you needed it. While it is nice to be skilled, to get items, to achieve things in the game whether solo or with a guild, they all pale in comparison to meeting such compassionate and wonderful people. I genuinely feel that being a dumb wayward kid playing an online elf simulator in my early teens would have been a waste of time if not for encountering the kind of people that make you want to be a better and kinder version of yourself.