This is causing me weird cognitive dissonance, since Galaga is FREAKING HARD.
I have to postulate a question for the OP; Does the game have to be single player, or can I always assume a community to play with?
Interesting - *Brotherhood *was the one I liked the least in the series (though I probably would replay it more readily than AC1, which really hasn’t aged well).
Gameplay-wise it’s probably the best, but the architecture is so uninspired and chock-full of asset re-use… which is a problem considering half of the fun of these games is climbing/parkouring across interesting cityscapes. *Brotherhood *is half-fields (with token broken columns), half ruins made out of bricks. The Vatican is different and fresh, but barred to you for most of the game. Even the Roman vestiges are copy/pasted for the most part. Yawn 
(Also I hated the wolfpelt Roman-inspired secret society - completely out of place in the setting, and doesn’t lead anywhere :/)
Well, I’ve been playing WoW since 2004, and I haven’t gotten tired of it yet. I’d say it’d be my favorite forever, but since it’s a pay-to-play mmo, I have no idea if it will even BE around forever.
So knowing that, probably the Portal franchise. The plot is just so interesting and hilarious, and the gameplay SO innovative with practically endless possibilities, and the songs are so, so catchy.
I agree that they could’ve done more with the Sons of Romulus (the wolfpelt guys) as they were an interesting ally to the Borgias and would’ve served the plot well. I get the feeling that they had more planned for them, but ran out of time.
Maybe they’ve reused map objects, but hey, they had to rebuild every inch of the largest city of the ancient world. And they did it in a year. Add that there was a lot of new stuff (horsemen, arquebusiers, multiplayer, etc.), I’ll let Ubisoft Montreal slide on that. 
Eh, Galaga just requires some memorization, you have to know where to be and right when to fire. I know on the Atari 7800 I eventually got to the point where I played through the entire game twice through in one sitting and got tired of it (I would have been 8 or 9 at the time)- but once you get it down you tend to kill virtually all the baddies before they sit at the top of the screen and do the one by one dropping animation, and you get all or almost all perfects in the bonus stages.
Oh, and in response the OP the only games I never really got tired of were ones with really good multiplayer - Call of Duty MW2 and Halo 3 multiplayer were the best for me.
Grand Theft Auto IV
And Civilization IV. Ok that’s 2, so sue me!
Terranigma on the SNES. I never played Chrono Trigger (I’ll get round to it!) but this game affected me deeply.
You HAVE to play Chrono Trigger. Really, you do. Great story, great gameplay, lots of optional sidequests for different characters, and the characters do have different styles.
I think my favorite character is Frog.
Forsooth! Thou hadst made a good choice!
The characters were cool. Only part I didn’t like was that for me, Ayla was mandatory because she was the only one who could steal, and I didn’t want to miss anything. Didn’t use Lucca much.
That game also has like 30 endings, although all but a few require you to beat it once and run New Game+
Dark Cloud. The dungeons may be repetitive and the story a bit weak, but the game is pure nostalgia for me, and I’ve replayed it several times over the last decade.
Chrono Trigger is not even in my list of top 5 favorite JRPGs (probably top 10 though), but it’s still amazingly good. I thought Cross was even better though.
You can choose any game you want, but if you pick an online experience I guess you’re just screwed when the servers go down:)
Discs of Tron arcade game. Environmental cabinet, with new or completely refurbished push/pull spinner and joystick.
I’d say Rome: Total War. The graphics are dated now but not terrible at all. The music, artwork, game mechanics, setting, are all just perfect.
Nothing like killing barbarians in the north for the glory of the Empire.
SPQR
Alpha Centauri.
I love it so much that I uninstalled it from my computers.
It just took up too much of my time.
Cross was almost perfect. There were way too many playable characters, though. And each one had to have its own distinctive speech style, which got on my nerves in some cases.
I did enjoy the alternate timelines story. I ESPECIALLY liked the item you got on New Game Plus, which allowed you to put someone else in battle instead of Serge, so that other characters could level up, too.
Civ II. I’ve been playing it constantly since it was released and never get bored of it and am still sometimes surprised by the AI.
You know there is an exploit in Galaga that prevents the bugs from shooting at you?
[spoiler]In any level, keep one of the left or right-most lower “bugs” on the screen forever. He’ll just go down and down in the same patterns shooting at you. This is easy to handle, your worst enemy is boredom as you must watch him go down for 10-15 minutes.
When he is no longer shooting at you (give it three go arounds at least), shoot him (the first time I did this I ran into the thing. :o )
The rest of the bugs will no longer shoot for the rest of the game. [/spoiler]
This worked for the standup version back in 1982.
Did you ever play Age of Wonders and/or its sequels? Definitely worth picking up if you haven’t.