One thing I like to ask people who mention "too many memes" as a complaint

How is anything in Undertale a "meme"? A meme is a "an idea, behavior, or style that spreads from person to person within a culture"

That means any joke or observation or action can become a meme. "Three wolf moon" t-shirts was a meme and that was just a shirt company selling the same kinds of shirts they always sold. Memes can't be "created" in fiction. You can create JOKES in a game that BECOME memes. The hoop rolling scene at the end of Portal 1 was something that the dev team thought would become a meme, they nicknamed it "hoopy" but that didn't come to pass. They were surprised when the companion cube became ridiculously popular at the time.

It's popular because A: It was created by a guy who created Homestuck and B: It's a subversion of games a lot of people my age played when they were younger. It subverts audience expectations (if you actually play the game to 100% completion and get all the endings, as we would in the Snes era of games) for the genre by personifying your enemies to be more "human" so that you question why you should be killing them to become more powerful. Bullet hell, for a lot of players who liked Undertale, is actually a 100% new concept to them. Bullethell is a super niche genre. THAT and the game doesn't just play like a bullethell, it eventually plays like a platformer and Frogger as well. Besides that, it has a lot of puns and solid humor that is satirical of anime fans (it also has subversions of tropes in a game already about subversions. A lot of characters are NOT what they appear to be. Even Asgore, the "Final boss" is the exact opposite of every Final Boss you've ever seen in a videogame).

Besides that, the main cast is also the bosses which is craaaaaaaaaazy for a rpg. So it's a combination of a lot of things for why it suddenly exploded. So, reiterating, A: Homestuck community B: Game gets better the more you understand about snes to playstation era JRPG games.

It's also $10 and that is cheaper than a full movie. You can't complain about the price of anything that's around $10, that's $5 less than a bottle of Bacardi gold and half the cost of a paperback. 

I'm not sure if it's the best game ever, but it's "up there" in solid storytelling (that isn't to say the gameplay is the best ever). It's also a rare type of game that tries to spread a message to the people who play it, namely determination in the face of adversity, forgiveness, selflessness, cruelty. One example is that Determination in the setting is seen as both A: A force for achieving your goals and B: A force for achieving REALLY BAD GOALS NO MATTER WHO YOU HURT. The reason I mention this is I've met at least one Undertale player who literally revolved his entire lifestyle around the things he learned from the game. There's similar things with the Dark Souls community where Dark Souls cured their depression, but that's another story.

Another cool thing is the game basically sees you as an outside influence. That character on the screen isn't you and the game acknowledges this. You're an eldritch abomination to the inhabitants of that world. You can reverse death, delete the entire timeline, and even kill someone over and over and they might mention it.

For reference, games I consider to have really good stories: Spec Ops the Line, Planescape Torment, Bioshock 2 (IT'S GOOD OK), Portal 2, Undertale, Metal Gear Solid 1, Metal Gear 2: Solid Snake (MSX game), Red Dead Redemption, Psychonauts, The Last of Us