Personally, I think "masterpiece" is outdated, overrated and overused too much, at least as far as general gaming goes. If I read "FFXII is a masterpiece", as I have many (many) times already...Okay, if you're talking about Matsuno's repetoire, yeah I can see that. But if you're talking about among videogames and RPG's in general...It sounds more than a little ignorant, especially to people who generally don't like Matsuno's games. Same thing with Halo or Grand Theft Auto supposedly "raising the bar" for the previous generation of games, which just made me cringe every time I read it.
Very rarely would I use masterpiece to describe something in general, as opposed to artist or even company-specific. I think "classic" would be a much better term. If I were to sit down and define a "classic", it would simply be something that's fun to play, looks good, sounds good, is well-designed, offers some innovation to the genre it belongs to, actually holds my attention from start to finish (and not in the "good god, I can't wait until this POS is over" way) and has stuff that makes me want to keep playing it. It's gotten incredibly rare to find games that meet that simple criteria.
And what the hell...Classics that come to mind:
Mario 3: The perfect platformer in my mind. I wish more platformers nowadays would take cues from all the stuff this game does right (especially the play controls).
Megaman 2: For a game that's as easy as this, I admit I play it again and again for the beautiful aesthetics and tight play control. I would also regard MM1, MM3, MM4, MM7 and MAYBE MM8 as classic, but no doubt MM2 is the best. Also see: Megaman X1/X4/X5 and MM Legends 1.
Contra/Super C: See above, minus the "easy" part for Super C.
Guitar Hero: The concept is simple, the execution is the fun part that gives it longevity, especially if you dig the music. They can milk it for years as long as they keep the core gameplay intact (eyes the amount of Bemani games in Japan). Adding more junk like the guitar battles against the CPU in III...Not a good idea. Not what makes the game fun or what drew people to it in the first place.
Tetris DS/Puzzle Bobble 2/Puchi Carat/Tetris GB/Pokemon Puzzle: Dreadfully simple ideas, unlimited replayability. The 1P modes in the two Tetris games are like two different games and I wish Pokemon Puzzle had online (instead of Planet Puzzle), but it's basically the same game. The 1P mode and graphics/music of the Puzzle Bobble 2 are my favorite of the series.
RBI Baseball 2/Tecmo Super Bowl: Say what you want...These are more fun than the most realistic simulations you can make to me. It'd rock if they remade these every year with updated stats/teams. Everything I expect from baseball/football games.
Final Fight/Double Dragon/Dynasty Warriors 4: You punch stuff, you throw stuff and you try to stay alive. Simple, but effective.
Cardfighters Clash: They made a fun video-collectible card game. Go figure. One of only a few games I've given over 200 hours to.
Galaga: Still amazing how a simple idea and a perfect execution go together. Shoot bugs, dodge beams, memorize patterns.
Burnout 3: Simple concept, braindead physics...But man, is it fun to wreck stuff.
Klonoa 1 and 2: Check and check...Not much to see here except platforming perfection (aside from being a bit on the easy side...Klonoa: Lost Levels would be VERY welcome).
Marvel vs Capcom 1/Street Fighter Alpha 2/Guilty Gear XX#Reload: My favorite fighting games and the ones all others have to live up to.
FF4, FF6, FF7, Chrono Trigger, Valkyrie Profile, Star Ocean 2, Ys VI, Suikoden 1/2/5 and La Pucelle: Same deal, but for RPG's.
Not all, but those are the most obvious ones when I think "classic".