The 3DS game. The main thing it had going for it was that it was a portable smash. Now Ultimate has that covered. All Smash for 3DS is now is an inferior version of Smash for Wii U with the same mechanics and characters but inferior controls and graphics. Every other game in the series has a unique feel and personality to it. Smash 4 in general lacked personality in its presentation even if it had a ton of great content.

as far as modern relevence goes, probably the original. I haven't seen or heard of a tournament for it in years, Tourneys are about the only way older entries stay relevant. I think alot of people are misinterpreting the question as "which game will leave behind the smallest legacy". Which is fine since that's a fun thing to talk about too. I'd say wii-u/3ds will have the smallest legacy. a small install base (comparatively) and terrible single player will always leave this one lacking to me.

I can agree from a competitive side it is the best version ever and really good at being that, but I'm not really down for aggressive play over defense, it reminds me of how street fighter 5 got further and further away from street fighter 3rd strike which had alot of tech that rewarded decisive defense moments to lead into offense, but thats just my preference, not really a knock on smash. 

I disagree that it's "sillly" to complain about modes that have been absent for decades, when ultimate is bringing back characters and stages that were absent for decades.  Brawl had a set of standard break the target stages that all characters played on,  something they could have brought back, doesn't have to be 80 stages. Also every smash had something fun and new, but ultimate hasn't done that yet. Spirit mode was ok, but it's more "event mode" under the guise of a spirit grind to unlock characters.

all-in-all, maybe its possible I'm growing out of smash bros or my taste of gaming is changing, but I'm just not eager to play smash everyday like i use to.

It's pretty close between Brawl (didn't like the slower more floaty physics) and Wii U (basically a weaker, more stripped down version of the current game). Never played the 3DS version but I'd imagine it'd be in the running for most irrelivent as well.

Smash 64 has the nostalgia charm, and there's a certain appeal to the simplicity
Melee was the perfect balance of simplicity and complexity, plus the mechanics are tight and solid
Smash for Switch is just chock full of content, it's the ultimate package with the slickest graphics, and it's cool that you can play a full blown Smash game via home console and handheld.

I would say smash 1 and 4(3ds). Even though smash 1 was the one that brought this magnificent beauty into our lives, it's rarely ever mentioned and no one talks about it. And the 3ds version is just as irrelevant. At least the Wii U version is talked about now(it'll disappear as well in the future). I don't think people will ever stop talking about melee, brawl or ultimate. Funny the ones with actual names are the most relevant, at least to me.

To the people saying the 3DS version, I have to bring up Smash Run. What part of Smash 4, between either 3DS or Wii U, do people mention as a feature they want to see brought back? It sure as hell isn't Smash Tour. That's the only thing that really stood out on the Wii U version that people talk about as a definitive new feature to that version, aside from maybe 8 player matches, but those are in Ultimate. Smash Wii U is 100% obsolete now. Especially once 3.0 comes out and we get the stage builder and home run contest modes, there will be nothing people are missing from Smash Wii U. Sure people want a Break the Targets game, but they don't want that Angry Birds nonsense we got with Target Blast. They want Melee's Target Test. Even 3DS had something more like what we wanted with the Break the Targets Challenge Doors in Smash Run. Smash Wii U is now an entirely forgettable entry in the series, while 3DS has Smash Run.

Now sure, if they bring Smash Run to Ultimate, Smash 3DS will be a lot more obsolete too, but one thing it'll always have over Wii U? It's not on Wii U. I mean do you really want to drag out your Wii U to play an old Smash game? There are things about that game's meta that I like, but I can always just do it on 3DS, which is easier to take in and out of storage, in fact mine isn't in storage yet, I still regularly play my 3DS and will probably play Smash on it again at some point. It's also portable and can connect with other players locally, which Wii U can't do, and while Ultimate can, I'd much rather not because I'd rather play with the 3DS' controls than the constantly breaking Joy-Cons on my Switch. Damn control sticks can't handle action games without breaking and starting to have that drifting problem. Until Nintendo fixes the Joy-Con stick drift issue, 3DS is the better portable Smash. Indeed, while I use my Switch in portable mode, I don't take it out of the house much, as it doesn't fit in my pocket. So in that sense, even if Nintendo does fix the Joy-Cons, 3DS will always be the more portable Smash to me. As such it will never be irrelevant to me.