Day 1 Impressions
Smash Ultimate is really good. It's extremely impressive that Nintendo has kept the Smash formula fresh on the 5th entry. At first it felt off like the game was laggy but as soon as I unlocked Inkling this delayed input sensation perception ceased. My favorite character right now is Simon Belmont. The assortment of moves he does with the whip are phenomenal. Ultra stylish. You guys absolutely have to play this game as soon as you can, if you aren't already.
Day 2 Impressions - Post 200 Battles
Easily better than Brawl (or its mod Project M) and Smash U; The fighting is sped-up, and the characters feel more responsive as a result. The graphical flourish of the game makes those past two games look dull. Seriously, the colors are extremely vibrant. The new characters feel great to play as with Ridley being my favorite newcomer. Heavy characters (i.e. Donkey Kong) are much more imposing now in terms of strength and stature / size. The level 9 AI is suped up and has handily beaten me several times over (King K. Rool is brutal). The AI feels closer to Street Fighter AI than past Smash games with how the computer makes you encroach their space to effectively fight against turtling. It's a ton of fun unlocking characters as it's a grabbag of who you'll fight and whether you'll steamroll them, go blow-for-blow barely squeaking out a win, or just get completely demolished.
On Stages - there is one stage in multiplayer, although it first appears in Smash U I only messed around with it now, that is pretty chaotic and reminds me of something you'd see from a Rampage game (or the Outfoxies): Wrecking Crew stage.
Find Mii stage [edit: Actually this is a stage that was exclusive to the 3DS Smash Bros.] is a really nice entry to the stage roster. Bouncing people off the hanging cage or slamming them upwards while underneath it reminds me of the good times on Hyrule Castle stage in 64. With stage hazard on the cage swings back and forth like a pendulum, which is an awesome dynamic for a stage to have. There also is some dragon. On hazardless the cage is a lifeless unmovable solid structure and the dragon is gone. Would have liked to see the option to remove the dragon.
Smash 64 and Melee are tough enough to compare against one another without throwing this fifth Smash entry to create a third comparison. (Each game can standalone.) Melee is pretty much the sequel anyone could have asked for coming from 64; it's super-responsive and the characters can move about at a much faster pace and do tons more cool shit, and then you have all the new characters, items, modes, etc. Ultimate is tinkering with the formula and finally getting a 3rd type of Smash Bros perfected in terms of feel of the game. It oddly makes Smash feel closer to a traditional fighter, at least as close as it's gonna get. In terms of pure fighting (1v1, no items on) I'd go Melee>Ultimate>64>>>everything else. (I really don't like placing 64 that low.) Pretty much Ultimate is a must play for anyone who has even the most mildest interest in Smash. As a fan of the series, it quite thoroughly surpassed my expectations.
Day 3 and Day 4 - 300+ Battles
All characters unlocked. I'm considering putting 64 above Ultimate in terms of plain 1v1 no item fighting.