EQ1:

Class - Race difficulties.

True Alinment system.

XP Loss and deleveling.

No quest logs

No AH

100X more content than you see today.

Day/Night cycles that are different gameplay.

Open ended dungeons.

Slow leveling.

True character development.

Almost a true PnP adaptation MMORPG. Todays games lost the reason for MMORPGs to exist.

WoW for being the first and the longest engaged (6 yrs).
TSW for being so refreshing that I bought a lifetime sub.

Those you played daoc from day one know the game was an unbelievable step forward that has never been seen agian.

I cannot tell you how i wish this game was still alife (much in the way of nge daoc's current state is a game i do not know).

So yeah DAOC easy, doesn't even have rivials for me.

I will say that wow BC was amazing for raids

I will say SWG pre-nge was amazing for freedom (still feel JTLS was one of the best x-packs ever)

eq1 bwas good uo was good, i preffered the muds at this time however, they were deeper (this is not an attack understand muds were amazing).

Eve rep also. (actually still playing)

ps. Someone mentioned atmosphere , oddly enough daoc had a amazing feeling to it. to was like i was in the hihglands, or a deep forest or in a lad of magic. just odd, i rarely get that feeling today.... maybe it too forced.... maybe it had to do with just walkign up to a very small tower and the music changing to some thematic track.

The first is always the best, so for me, AC1.  That was my first adventure into MMO's and the open world environment, massive numbers of people, and actually TALKING to people around the world was amazing.  I have yet to find a loot system and leveling system similar other than somewhat from AC2.

Second on the list was definitely AC2.  It fine tuned some things, made mistakes at others, but now that I've played it all weekend after the re-birth of the game, I am finding there is much to be desired in a 10-year old game that modern MMO's have yet to deliver.  As empty as the world is, it feels more alive than anything WoW or other MMO's have acieved for me...if they can improve the graphics and some of the gameplay, it coudl be awesome.

Anarchy Online

8 years full of fun and glory.

Face of Mankind

A political course in things of factional enforcements.

Yeah, PvP ^^

SWTOR

I actually play this, waiting for the AO engine.

SWTOR has a entertaining PvP full of tools and whistles, but would Age of Conan be sci-fi themed, i'd play that.

I like that kind of combat system for PvP.

There is ofcause hundred other games i played for some months, but these games above gave me the most immersion, fun and challenge.

Asherons Call.

I could play the way I WANTED to play. I on PURPOSE made a chacater that could swing a sword, use War Magic(magic that did damage), had NO melee defense(so took full damage) and could cook. A character that was a total gimp, not really good at anything but really great at parties. It was an open class system that allowed you to train what you wanted with limited points to place of course.

Better yet, it wasnt a traditional combat game. You could, with decent skills trained literally take on 20+ mobs at the SAME TIME and live, with enough health to go right to the next batch of mobs. Actual high ACTION combat. then on TOP of that, there were attack types. With melee you could swing high, middle, low. There was Stab, slash, blunt, acid, lightning, fire, cold based attacks and every mob had a weakness to one or more making swapping attack types a vast expansion on combat options.

It would have to be WoW. 6 years of playing one mmo consitutes the best for me. But the WoW days are long gone.

Honorable mention would be GW2 and Rift.

1. EverQuest II

2. Dark Age of Camelot (pre-ToA)

3. Vanguard/GW2 (tie)

1) Underlight

2)Meridian 59

Overall gameplay and systems, PvP/PvE balance, crafting, housing, cities - All signs point to SWG.

Stability, polish - WoW, as much as it pains me to admit.

Server architecture, unique gameplay - EvE

my 1st MMO was runescape, but now it not worth to be played anymore, IMO.

after I started to hate grind and picked up GW (all 4 games), most I liked I guess was Factions as it is very dynamic game, shame ANet never updated or upgraded GW. PvP used to be tons of fun too.
GW2 is far to be that fun to me, seems something missing but I can't really tell what, may be need to play more.

WoW gave me tons of fun! I really love this game, only dark side here is monthly payment,
most because I can't even visit my account if I have just little time and will not pay for nothing. Still think it was the best game I've ever played.

LORTO I liked but got to the point when needed to pay to unlock further features and payed WoW instate. =D

Forsaken World gave me lot of free fun as it WoW-like game, well it not that large and polished, but it free. =D

Well...back in the days i used to play a lot in pirated servers...

It was mostly MU,L2,WoW,RO...And all of them had tons of players in my country and it was awesome and big community with IRL meetings and stuff.

RO was over L2 and MU....But i really loved WoW from day 1...thats when i started to leave pirated servers and become legit :p.

Sadly My WoW love ended with TBC...i dont like WOTLK CATA or Pandaria.

Other games i loved were Shattered galaxy...a kinda MMORTS 2D and Face of Mankind beta...The game is heavy in roleplaying and i beta there WAS TONS of players...always raids and the game was a blast....i would pay a kickstarter for FoM 2 well made.

Also Freelancer was fucking amazing....best space game.

And i think those were my fav games....In Wow and Shattered galaxy i wasted years on those lol...

And now im really enjoying Planetside 2...but im missing the roleplaying from FoM....I think i really love MMOFPS cant wait for Defiance in 2013.

Everquest 1. 

To this day, nothing has matched the fun that I had running that world.

WoW is hands down the best I've played. There's a reason so many continue to subscribe...it's called "fun".