Ultimate > Wii U > Melee > Brawl > N64 > 3DS
I currently rank Ultimate as my favorite because it's so new and exciting. All of the new fighters are really fun to play, it's cool to have almost all of the stages back, Spirits are a nice concept and it's fun to see the references in their battles and to use your favorite characters Spirits alongside your fighters, and stuff like unique end bosses for some Classic routes is a nice touch. I've found myself getting a better feel for certain characters that I liked to use but wasn't as good at in previous games like Lucas and Roy, so that's pretty great. Some bits do feel rushed and some things disappoint me, but it's overall a solid, enjoyable game.
Super Smash Bros. for Wii U was great for me as it was the first game in the series where I could play online with friends. I had a ton of fun doing just that, and it was fun to find weird Miis whose QR codes I could scan and use to make people laugh. This game was the first in the series where I chose a main, Ike, a secondary, Marth, and really dedicated myself to getting good with them and the other handful of characters I'm good at compared to the rest of the roster like Mario, Peach, Samus, and Zelda. I sunk a lot of time into Smash Bros. for Wii U and don't regret it a bit, it's fantastic. I almost ranked it as equal to Ultimate, and that probably will become more true as time passes, but for now it's in 2nd Place.
Melee is next. Aside from Wii U, this is the one that I put the most time into since I was a kid and had more free time to waste. The speed was a fun and unique aspect and the roster was solid. I also played this one a good amount with local friends back in the day. Oh, and it's the introduction of Fire Emblem to the Smash Bros. series with Marth and Roy, so that's great as far as I'm concerned.
Brawl comes after that. Subspace Emissary was an enjoyable mode and it brought in some of my favorite characters to use like Ike and Lucas, both of whose games I love. I was very disappointed with the removal of Mewtwo and the addition of Lucario in their place. I was pretty annoyed with that back in the day, even if the Aura aspect adds some more interest to the playstyle. I've never really been a fan of Lucario in Smash Bros. or in the main series Pok?mon games and its popularity feels a bit baffling. To get a bit off that tangent, this was the first game in the series to actually remove characters that were previously in, so it felt in some ways inferior. But hey, to end on a positive note, it was cool to battle bosses like Porky, Rayquaza, and Ridley.
N64 is next. I played it back in the day when it came out and still remember seeing
the commercial on TV as a kid. It was fun for its time and I played it with friends, but nowadays there's nothing to come back to really. It was great to see Ness as an unlockable character at the time since my brother is one of the people who actually bought EarthBound when it came out and I watched him play it. Again, good for it's time, but now a bit too archaic.
Finally, in last place, there's Super Smash Bros. for 3DS. The game had some good aspects like its stage selection, which I honestly greatly prefer over the stages present in Super Smash Bros. for Wii U. But honestly? After it came out and I saw tons of reports of people breaking their Circle Pads by being too rough with them while playing Super Smash Bros. for 3DS, I became quite reluctant to bother playing it and, while I initially bought it to get as early access to Mewtwo as possible for the Wii U version, almost as soon as I got Mewtwo I sold the game to buy something else instead. Additionally, the only controller I feel comfortable using while playing a Super Smash Bros. game is the GameCube controller, and that obviously wasn't an option in this case. Also, even though I didn't ever play as them much and even still to this day don't, it was the 3DS port's fault that Ice Climbers were removed despite being completed for the game already. The game is probably fine, but it ultimately gave me the least.