I deliberately left it vague. Its a tough question I know. However taking everything into consideration would say either Baldurs Gate 2 or wizardry 8.
Lands of Lore is a no-no (though a very good nomination onetheless) because of the hectic nature of combat (cliclty click click) and two very annoying BADLY designed places:
1)the white tower where you need to run like crazy through the ghosts and use the stupid cube
)The place with the circle traps, where your atlas gets taken away. That was VERY badly designed. It makes no sense on so many levels..
Start Trail, while my fave game is also not the best, because of a few poorly designed things such as too frequent onset of diseases, way too many red herrings(what did you ever need those red keys for..)
Note I personally liked the frequency of diseases; thing is they could have been implemented better. They are not near that frequent in real life in ancient times!
Ultima 7: Combat. poorly designed. Follower mechanic, also poorly designed. No wonder in u7-II they made it so you could not die. (which IMO was the worst deign decision ever and partially ruined an otherwise great game)
EOB series: See LoL. Also, ad&d ruleset not implemented correctly. Good games though, lol 2 especially.
KOTOR: Another gem. However, combat design does not scale very well; slow and the beginning, uber fast later. Thing is, while some say this emulates star wars; its just not possible to have the epic lightsabeer duels of the movies in the game! They had to have Malak cheat (using the clones for energy) to prolong that duel. The duel with Malak's apprentice bandon was very disappointing (over in 4.8 seconds!Or so it seemed.) This is the fault of the d20 starwars system though. SAGA is better, would love to see a SW game based off that. Great gamne but not the underlying ruleset could have been better.
NWN: Too many flaws. Poor design indeed, at least when compared to Bioware at their best. Sorry voly!
IWD: Design choices with regards to npc's interaction could have been better. Yes I know the game is meant to behack and slash. However when deciding on the best designed crpg ever you need to be ruthless! Great games though!
bg1: Empty in too many places. side plots too dry. (Nashkel mines was a yawn-fest.) Great game though.
Wiz 7: Too many random enounters. AAARGH!
Xeen: This is actually a much better contender than some might think. Sadly, the NIL npc interaction (forget dialogue tree's, you don't even have a parser. All conversations are 1 way) couped with the way levels and pcpower scale make it not tobe.
MM 7: Another strong contender! Sadly, combat could have been better. Towns could have been better. NPC's could have been better.
Jade Empire:Fantastic epic but... NPC's cannot harm enemies for some reason! Even if they punch and kick for aeons!
Arena: IMO the best of the series and a damn innovative game at the time. Spell creation was a new and amazing concept! Waving your mouse to attack with the sword/weapon/hands was fun! The gothic atmosphere was immersive. Yes, it was perhaps, in some ways an ominours foretshadowing of things to come (i.e. action-rpgs since it was the first ever) Sadly, the poorly designed NPC interaction and NPC's themselves as well as the too fast nature of combat keep this far from the end spot.
Fallout: Whats the point of having recruitable npc's and a fantastic combat system if you can't control those npc's in combat?
For obvious reasons, any game by troika and/or obsidian will not feature in a 'best designed game' thread :D
BG2: Deep NPC's. Deep plot. Rich setting (not due to the FR background, no BIO made the game setting itself quite rich due to the tons of books and lore. Even mundane weapons and gems had rich backgrounds.) Fantastic graphics. Spell effects were awesome and quite elegent.
Character sprites, even for the time could have been better. On the rich tapestries of the background they really looked like literal paper dolls with stiff movement. On the plus side they showed weapons and armor rather well.
Quests were above and beyond the best designed thing about BG2. Thought provoking, with tough choices and great dialogue. Sure, they did not have fallouts independance but they more than made it up with depth and meaning. ALso, quests got epic as the game progressed. You got the feeling you were actually taking part in making and shaping the history of the world you were playing in!
Enemies were cool. Irenicus is still IMO one of the best villains ever in crpg's. Totally badass without being completely cheap. Firekraag just came out of no-where (so glad I did not spoil myself...but hey. Forums were still a new thing in those days) I vividly remember just staring at the screen at this huge red dragon and thinking...(FUCK! What a kick ass battle this is going to be!) And I beat the dragon right then (instead of doing other quests and coming abck) though it took around 3-4 tries.
I dowish they had some how implented an animation of a flying/hoveing dragon but hey.
Lastly, musci. Best.Music.In.A.CRPG.Ever! Period.
Please don't argue this. Its epic, its deep, its increadible and no other crpg or video game can match. (though some come close)