When i answered that question the answer wasn't Guild Wars, even though i have played this game more than any other game for the past 20 years.
I then asked myself why...why wouldn't it be the most exciting and memorable?
My most memorable was a game which used a real world currency system. You could put money in and take money out. Withdrawls depended upon being successful.
No, i am in no way suggesting such a scheme.
Basically, the loot system in that game was different. Most creatures would drop some skin, paint, degraded armour, weapons, furniture (on larger creatures) and small amounts of money but would only be worth very little.
Some creatures would drop slightly better things or a larger amount of money (around three times a week i would find one and these would be comparable to a rare drop in GW) but it was the search for that "super-drop" which provided the excitement and the drive to continue playing.
Players could get along fine without finding one of these super-drops.
They were so rare, i killed literally hundreds of thousands of creatures before i found one of these drops. (i'm sure there was some coding behind my probability of finding one...like "spend X amount on ammunition, armour/weapon decay" and the chances would be greater, but who knows?
Anyway, when it did happen, it was amazing. There would be a global message telling everyone of my success and my character would be engulfed in a magical swirling of stars to the sound of trumpets and cheers.
I had messages coming in from all my firends asking what was in my find and if i had got X weapon and if they could buy it off me.
I still have all my screenshots of my finds, even though i haven't played that game for over six months! They are still held in my mind as some of my most exciting times in front of my computer!
So here is my suggestion.
Add super-rare but amazing and fantastic drops to the loot system.
The lower level creatures would be harder to get a super loot from. The odds would be so small, it wouldn't be worth hoping for it. If it did happen, it's contents wouldn't be 5000 golds worth of 8-10 crappy shortbows... but max-damage golds , runes, dyes, animal parts and huge piles of gold instead.
Getting a super loot from a high-end creature would be more likely but still extremely rare (equivalent to three times the rarity of black dye would be a safe assumption).
These high end super loots would be comparable to any other super loot but may contain max gold versions of the "best/rarest" items in the game aswell.
Some loots would contain "near-unique" items.
When a super-loot occurs, each person would get a copy or near copy of the loot and this would promote team-playing since your chance of finding one would be increased.
Now, ArenaNet, you know what unique means.... None like it, one-off, "i'm the only player with this" etc.
Where are they? Greens are not unique at all.
I know you have to keep a balance but where is the harm in one or two people having a 16-23 sword or a 6:1 vampiric bow or a 17 armour shield? (OK, extra damage on weapons isn't the focus of this suggestion but it's worth considering at least).
Where is the harm in people running around with weapons which they can rename to whatever they like? This i would definitely like to see.
"I am wielding a beast master archers pwning bow"
Unique should mean unique and a super-loot system would give you the "opportunity" to reflect upon new items and ideas to keep people interested, as well as shortcomings with the overrated green items.
When a super-loot occurs, each person would get a copy or near copy of the loot (at least in terms of value) and this would promote team-playing since your chance of finding one would be increased. A side option to this would be to have one "massive super loot" which is equally distributed between the players in that team. So if there are 8 players, it would be equivalent to 8 super loots and then equally distributed.
There is still very little in the PvE perspective once the game is completed but adding this to the loot system would make people carry on playing i'm sure.
Perhaps you could add a hall of fame for the PvE side? If you found a loot of X value, you would go into the hall of fame if your loot superceded the previous position holders.
There could be a 24 hour hall of fame (which would change fairly often) and there would be an "all-time-high" board which probaby most people wouldn't get to be on.
When one of these moments happen, you are presented with a special loot window for screenshot opportunities.

It is in everyones interest to keep people in the game and keep Guild Wars exciting which is why i am trying to make these suggestions. Some people may not like them but i can't really see any problems.
All ideas that are ever implemented give everyone the same opportunities anyway.
Summary:
Add super rare drops to monsters which have a base value of no less than 5000 gold and the cap would be unlimited.
These drops "could" contain a truly unique item if found on a monster of high level. Perhaps a new colour for "unique" items?
Super loots can happen at any time and at any place...on a level 4 devourer or on Coventina the matron...wherever, and this has no effect on the item quality or amount but is more likely on high end monsters.
A PvE hall of fame system for the lucky people.
A global announcment telling everyone on ALL the guild wars servers/districts if you reach the all time high board.
A continental announcement if you reach the 24 hour hall of fame board or obtain a super loot that doesn't quite make it to the hall of fame but is still classed as a super loot.
Magical stars, sounds, trumpets and cheers surrounds your character upon getting a super loot...to make that moment memorable and exciting.

... id love to get a super drop THINGIE in this game...
give me more reason to randomly kill things.
