Even though the game engine and graphics are antique by today's standards, IMO it's still the best. I only came across one other game that got anywhere near it, and that was The Legend of Darkmoon (Eye of the Beholder II). It's head and shoulders above all the rest.The huge advantage that MM3 has over EOB2 is the degree of randomness built into it. You can play it again and again, because when you open a chest, or loot a corpse, with very few exceptions (and those are necessary), you never know what you're going to find. I'll never forget the first time I found a doubly enchanted obsidian weapon - it was an Acidic Obsidian Longsword. When you play the EOB games, you always find the same items in the same places. There are no surprises.
I was never interested in mindless shoot-'em-ups, or other games where digital dexterity ruled, and I found the impersonality of strategy games boring. I started out on adventure games - I was there when Colossal Caves and Zork were around - and I thought Space Quest 2 & 3 were among the best of those. But the writers started to have to stretch to present new challenges, and the games just got silly. I thought Space Quest 4 was a disaster. You start out having to catch a rabbit, and to do that you have to use a loop of rope as a snare and hide in exactly the right place - ludicrous!
I think the RPGs were the finest single-player games made, and of those, for me MM6 is pretty good, but MM3 is still King.
