Still, "perfect" is different things for different people - do you want monsters? Deities? Intelligent vampires? Horses? Cars? Space ships? High tech equipment? Cybernetics? Insanity tables? Advantage/Disadvantage lists? Gritty realism? Hollywood/cinematic action? Fantastic non-human races? Psionics? Magic? Classes and levels? Attribute-based skills? Points system? Alignments? All/none of the above?
What would be perfect for me would in no way be perfect for a person who wants Hollywood-style fights and one man holding off an army at the narrow pass - I'm not saying that that style of game is "wrong", it's just diametrically opposite to what I want out of a game.
Likewise "alignments" don't work for me - they never have and I always disliked them in D&D/AD&D - yet others prefer them.
Sure, I'd like to be able to say "hey, how about the party's jeep blows a rod out the side of the block and the only transportation they can scrounge up is horses" then reach for a book and voila! we have the stats for everything from a Shetland pony to a Percheron - I can't say, "Nah, don't need stats for old-style stuff like horses, swords and shields" because I don't know what random stuff the game might lead to. Horses exist, swords and machetes exist - I want the stats for them, or at least the ability to easily work them out. The ability to have/calculate the stats for things that don't exist but may well exist in a game - Plasma guns, aliens etc - is also important to me.
But other things - levels, classes, the actual combat stats and planar addresses of deities, Balrogs, alignments, grimoires and such - I can happily do without.
Others need such things for their games to function.