It's a close one between (Shadow Meld) and
(Scorpion Wire) but I think I'd say scorpion wire. These skills I love strictly for pvp or mirror 1 v 1 's because it is pure tactics at it's finest. There is a wild variety of crazy combinations between those two abilities that has you trying to predict the future of your opponents movements and one upping them with your own premeditated synergized combinations. Because these two abilities either counter or work with each other the thought that goes into implementing strategies with them should give clarity amongst those with higher wits. It goes without saying the amount of strategy, tactics, and execution will reward those who have in depth thought while those who have conceived smaller ideology, dynamics, and mechanics of the build will suffer tremendously.
I'll nominate Frenzy. It's a perfect representation of how GW was meant to be. One of the core concepts it was built upon was that a good team with mediocre bars ought to beat a poor team with meta bars. Skill choices were important of course, but execution was more so. Frenzy is simple in function but complex in application, allowing for skilled players to be rewarded with better results. That it's been meta consistently since day one and yet never been modified says it all imo.
Pre-nerf Shadow Form. WTF was they thinking lol.
FRENZY$$12'&'&'&pofazjofjazifjoazpj&'&j'^&'if&f&jjmojaazazazafkvsnc;;nsdsqdnazfn!:;*=))azaz
Xinrae's weapon 
Until today, I still can't find another game with skills that are even remotely similar to this, or as amazing as this versatile skill.
My favourite Guildwars skill isn't actually a Guildwars skill. My favourite is the skill required by the player to actually use the skills with seamless devastation, even if the skill bar isn't considered 'meta', just as Marty said above.
Using the terrain, cover, bottlenecks, understanding recharge times vs interrupts, knowing the way the AI reacts to what you're doing.... knowing which skill combination to use in any given circumstances...
In a nutshell, my favourite guild wars skill is contained inside the brain of the highly skilled squidgy bit of meat at the keyboard who completely understands the game mechanics and how to fluently use them to his advantage.
That's the only reason why I bought Faction for.
This and Assassin Profession.
What a simple question: Infuse health!
Jeez, the most satisfying thing in GW (imo ofc), use infuse health on an ally that is getting spiked , and save his ass 
"One to remove them all". With this iconic skill you can remove 7+ hexes/conditions. At 5energy/0,25sec casting time/10sec recharge PnH is still quite a viable skill. This skill is full of memories. Sadly it is not used as much as it was years before and now it rarely comes with meta, but I still love it and occasionally run it when I have a chance.
I would choose 2 skills. Lacerate is still viable and Edge Of Extinction is just a funny skill. And both are for people with balls daring to play pressure. Too bad spirits in general are weak atm compared to everything else.
Signet of spirits. I must have used that skill a million times.
Orders (Order of the Vampire)!
brings back memories of doing B/P runs through Urgoz 
[Second Wind]
Because of the major amount of energy it generates if you got a large energy pool and let exhaust rest between 40-50%.
It's fun to start a combat by immediately exhausting yourself by clicking and cancelling this skill, contrary to where you try to keep a low or zero exhaust.
oh, another skill that i really like was Selfless Spirit after it was buffed. but that only lasted like a month...thanks a-net 
Endure Pain, Saved me million times on my runs 
D-Shot !
(When you catch WoH in RA !)
I've only used Selfless Spirit once or twice.... I must have been taking a break when they buffed it. *Sigh*
"reduced recharge to 15 seconds; decreased Energy reduction to 3. Functionality changed to: "For 15..20 seconds, spells you cast that target another ally cost 3 less Energy." this made monking an over-aggro happy hm pug a breeze cause it was maintainable. a-net was not having non of that so they nerfed it a month later to being worse than it was before the buff.
Mine would have to be 
(Dwarven Stability). This has made my Chest Running Addiction so much easier, with or without Alcohol (in game or out of game hehe)
Ebon Battle Standard of Wisdom
60% chance on HSR!?!?!?! I mean come on! It's so amazing to see your slow recharging OP skill recharge twice as fast. I use this skill in almost all my caster builds.
Aura of the Lich Ive always loved to Minion master
Ebon Escape.
Hippity-hop, all the way to the birdie boiler. Noooo, not the birdie boilerrrrrr!
I'd have to choose EE because it's always a favorite of mine for dungeon-sc.
