Never had issues with Mario Kart 8 online aside from it being colossally boring. With Mario Kart, though, I find the experience is 4 to 6.5 fun-factor in single player/online - but jumps right up to a 9.8-10 when playing local multiplayer: especially with 6+ people.

I imagine it’s a similar deal with Smash, I had the N64 version, played the crap out of that multiplayer - but then later versions I played mostly single player and found them all boring/repetitive; aside from that adventure mode on the Wii version. But, it’s not just Smash, I find ANY kind of 2D fighter boring unless it is local multiplayer... Except Soul Calibur, the first one on Dreamcast, that single player mode was the bomb! But Soul Calibur single player campaigns sucked in part 2 and onward.

I play a lot of splatoon online, and it had feel just smooth for me, but i'm not an expert, and i'm still playing casually. The only thing i noticed is that there's seems to be a lot of people that seems to disconnect mid match or even before it begin

I played 1000~ online matches in Smash 3DS and never had more than a second or two of lag in a couple of matches on my shithouse internet (1mbps down, 0.5mbps up a couple of years ago). Splatoon on the other hand is beyond shit (even with the much better internet I have now,) MK isn't much better, but at least it doesn't disconnect. If Smash improves on 3DS online (which it damn well better) then I'll be happy. It would be good if they used servers for it, rather than the crap P2P Splatoon relies on.

Hopefully the upcoming paid online (which is the cheapest one among the other 2) Will make it better for you.

This. I get that it's cheaper but if it lags like hell and you have to pay for it it's no bueno. It also has to have system level voice chat. I myself don't need it that much but I get that others do.

For the avarage user the way of mario mart and splatoon is way better than smash (wii u). Its a trick, but they make them work seemingly more smoothly.
Sending messages to other players is what I would like to see in Switch games like Splatoon 2. Matchmaking in Splatoon 2 seems fine to me, apart from turf war and lowest ranks. The battles are surprisingly even most of the time for me and it feels like both winning and losing are possible, unless its 3vs4.
Hopefully they will make Smash5 online better and add more ranked modes.

I've never actually played Smash online myself, however what I would love would be a tournament mode, or potentially even a season mode where you can form a league with your friends and then face each other a certain number of times to determine a league winner (or perhaps do a playoff feature where a certain number of players advance to the playoffs to determine the champion). If there is a story mode it'd also be nice to be able to do co-op for that.

The amount of times in Mario Kart where I finished just before another player, only to find out that I lost to that player...

Nintendo has never gotten online right and I doubt they'll put in the effort to do it right with the Switch (although paid online makes one hope).

Well, to be honest it shouldn't work that way. We shouldn't pay in hopes of them making it better, they should have to make it better to convince people to pay.

I agree, but until recently Nintendo never even valued online multiplayer. Miiverse was a cute novelty but it was also yet another example of Nintendo wanting to do things their own special way instead of just incorporating what works for everyone else. I can only hope a paid subscription forces them to actually put forth a quality online service.