Since everyone is making Top 10 lists:
1:

My forever main, even though it's way harder now he's no longer Top Tier as in the other two games he was in. I'm having fun to relearn this character and proving myself and everyone that despite the nerfs, he's still a force to be reckoned with. I also love his speedy trickster playstyle and how you constantly are playing mindgames with your opponent. In general, just everything I want for a main character, despite secretly wanting buffs for him.
2:
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This character has always been one of my most used ones. I just love how dominant his advantage state is and the sheer speed combined with raw power. Grabs are also rewarding, and landing KOs this easy is just plain fun, especially since I don't have that with Diddy. The contrast between the two Kongs keep them interessting to play both.
3:
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Lots of nerfs, yet he remains powerful and technical. You always gotta read and predict your opponent a lot. It's more of a challenge now, but still one I'm willing to take. Quite combo based still, and he really teaches you when to be careful and when to be aggresive.
4:

He's crazy good this time! And my long hours of playing him starts to pay off. That new U Air is awesome, and the fact he can play his intended playstyle finally is something I appriciate. It's the same character as ever, but now with combos and actual stage control. I am not yet as good with him as I want to, but he's probably gonna be played more in the future if he isn't nerfed in the 3.0 patch and if the others aren't buffed.
5:

I liked her playstyle a lot in Smash 4, and once I got to play Awakening, I loved this character even more. Such a well written character, no wonder she was always so popular. Like everybody else, she's one of my effective secondary characters. But if she remains so dominant, she might be played more in the future
past (that one you'll only get if you played Awakening).
6:
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He's just a fun character. The trickery stuff of Diddy combined with heavy weight status of DK, and fun gimmicks to work with. Shame he was nerfed so badly. I likely will remain to play him, but as I said, just for fun and nothing serious.
7:
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I loved Breath of the Wild, and this new Link is improved so much.. how can't I play him? I always had a pocket Link. And the other smaller Links don't really work that well for me anymore. Or at least, Young Link doesn't, and Toon Link is still effective for me but lacks the things I find in this Link. Anyway, he's a blast to play this time around.
8:
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Ahh Ness, luckily you remained so faithful to your other Smash appearances, and seems to be slightly improved even. Haven't played the psychic wonderkid in a while, but I can't "unlearn" his playstyle when I haven't touched him for a while, and that's good. Especially for the long run.
9:
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Working on this character currently. He's just.. very explosive and matches really well in the playstyle I have discovered for all the other characters I play. He's a perfect blend, and easy to pick up and play. I also legitimately like him after playing Awakening and anticipate his entry in Smash before he got confirmed. Might become one of my "Top Tier mains" togheter with his daughter and Ike.
10:

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Am basically looking which of these am gonna invest more time in as the meta progresses.
Characters I'm still interessted in:
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: Powerful and explosive, fast and combo-based. Yes please. I could potentially make him a secondary. Played him for fun in Melee, and always wanted to pick up Pikachu again after Smash 64 but Pikachu never really connected with me anymore.

: Read above. Pikachu only has the benefits of being more safe and harder to kill.

: Greninja has a lot of the characteristics am looking for in a main, but he controls so slippery and his moves are hard to learn. Basically, he's too demanding for me currently.

: I need to learn 3 characters in order to play the Pokemon Trainer. I played Charizard for fun in Smash 4, and was quite effective with him, and Squirtle comes in pretty easy. Might look into this interessting character more.
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: Chrom, but harder to learn. Like Greninja, I might not really wanna explore him as deeply as I would need to in order to play him effectively.