I love MM's 6 through 8, hated 9, and tolerated 10 because it was at least old-school RPG. (The original Grimrock was supposed to be pure old-school, but I had to give up on it. My reflexes are simply too slow to get around that dungeon where you had to flip a switch, then run the length of the room to reach another switch (or something) before a pit opened up to swallow you. RPG's should NOT be twitch-fests!)
My favorite MM of all time, however, is MM3. It was pure genius for its time. AFAIK, it was the first to show facial expressions on the party's members. I laughed out loud when they stepped into the poison puddle, and turned green with their eyes crossed and tongues sticking out! No other RPG had ever done that sort of thing before.
Of course, I'm showing my age at this point. Is anyone else out there old enough to remember the old SSI Gold Box games, and its predecessors? I'm trying to remember: was it they or the M&M's that started the red potion-health tonic, blue potion-mana restorer color coding? Of course, SSI had RPG's before the Gold Boxes. The first one that I ever played was Phantasie, which I'm pretty sure was SSI.
P. S. Since I'm younger than my husband, I'm not the oldest RPG gamer around. But I think I'm the oldest FEMALE gamer!