Hey dude (Bestsamusever)- you seem like a nice, thoughtful guy, who loves the same game we do, so I'm going to ask you to trust me on something-
We get people in here all the time saying "I'm very good/the best" or "_____ is the best character/better than you give him credit for." You've been playing the game a long time (longer than I have), but unfortunately, playing solely with a small group of friends leads to inbred playstyles. If you played, as I and the others on this forum have, with a diverse and large group of elite smashers, you'd quickly learn that there are a number of techniques, situations, and whatnots that are virtually impossible to discover with only a few smashers. So the tl;dr on that is- We know what we're talking about. We really do. Samus and Link are balls. Jiggs isn't superbad because comboing into rest is quick and easy on most characters, and you really won't miss much at all if you're good.
On the stages- You could include every stage in the game, and Samus would still be #12. We choose not to play certain stages because they lead inevitably to "dirty" matches- that is, matches where instead of playing the game in its purest form, you're exploiting the stage gimmicks over and over again or else getting messed up by them- spending time dealing with things other than your opponent(s), in other words. That being said, playing on "neutral" stages has brought our games to the point where we'd wreck on the other stages too, should we deign to play on them.
As for the Free-for-alls- there's no way that smash is "meant" to be played; Sakurai allowed it to be customizable so that we the players could choose the best way to play. Following your logic, items would be allowed as well- and I hope that you see the folly in that option. So yeah, FFA's will end up with campfests and whatnot if people are really trying. They're good for laughs, but not the true test of skill that a 1v1 is. We do play a lot of serious 2v2 teams matches, by the way.
Oh and WOW at 10,000 hours. If that's true, you've spent like 2.3 hours a day for the last 12 years playing smash. Sir, I salute you; I probably haven't chalked up more than 1500 myself.
Why Is Donkey KONg So low???
Because he's slow and easy to combo and has a lackluster "approach" (that is, it's hard to get into grabbin' range using him) and he's kind of easy to edgeguard and doesn't have many combos.