It so much depend on circumstances...
(Do never belittle someone who had a hard time doing an area for the first time.

)
Today, for an experienced team, having all skills, knowing all tricks and abusing the monster AI fully, no area is too hard.
However, for a team only using skills that was available when the area was introduced, and not knowing anything about the area, it is a totally different thing.
- Two newbie lvl 7 players stumbling into the charr shrine in Pre-sear. That is hard!
- The first teams into UW and FoW probably also had a really tough experience.
Today there are more skills and a lot more experienced players, so new "elite areas" are conquered faster and made "public domain" not very long afterwards. One main reason is the "predictability" of the areas. A new elite area very soon breaks down to "stand at point A and use skill 1, then move to point B and repeat". Making "area loads" choose a random combination of several predefined monster spawns would have changed that a lot.
My hardest one was doing the Deep for the first time.
We had a balanced team where no one had done the area before or really knew what was coming. We had heard that knockdown was required so one of us had that. Of course only the knockdown team managed to get out of the first room, but they managed to save the rest. A few times when we got stuck (like the "stand on marked spots to open door"), one of the player checked the wiki and gave hints. We really struggled through the scorpion room, had those teleport-knockdowns every 5-10s, but finally made it through. The rest went really great up to Mallyx. He gave us a really hard time since we only had a single necro enchant removal with too long recharge time. When we finally used the knock down and got the massive spawn we almost wiped, only the bipper survived (me). I had to solo a couple of shadows to rebirth a ranger so we could take out the rest and ress the whole party. When we tried once more with the same result we gave up after almost 5 hours, declaring Mallyx as a "really tough cookie".
But that is also the hardest area, and longest trip I have done with an "on sight" team (not counting pretty short first trips into UW/DoA).
My deepest respect to all who do an elite area without any knowledge before hand!
Some notes for developers:
Biggest adrenaline rush: First time at Chaos Plains UW, with an unprepared balanced team. That interrupt/hex fest was a blast! How we survived it is an enigma, but it felt really good afterwards.
Biggest letdown. Killed by the massive spawn when whole party entered the room in Urzog's Warrens (after 3 hours of struggle).
Most boring: Pull and kill loads of almost identical groups in Urzog's.
Most fun: Central plain in FoW with tankless balanced party using kiting to a combat trapper (oath shot, barbed+dust trap, throw dirt) to get rid of melee.
My biggest dislike: The exclusive team builds needed for DoA. To do that area with different kinds of balanced teams is really, really hard.
Most retries before finishing: Mirror fight in desert. Had no clue of any tricks. Tried to out-damage with a necro blood build. Did it in 7-8 tries.
Biggest struggle before knowing the trick: Flame djinns in Sorrows Furnace.
Best added game feature for elite missions: Reconnect!
I will always remember: First time through all areas of UW with an at the end rather reduced (damagewise) party - 2Mo, 2Me and a bip-necro, after an 11 hour ordeal, 4 hours to get the party together and 7 for the actual trip.
Regards,
Cloudbunny