So, here's my early top 5 (main, 2nd main, and backups) for Smash 3DS. Keep in mind I haven't played with every character yet:

Zero Suit Samus: ZSS was one of my "rotation" characters in Brawl. I'd use her quite a bit with friends, but never really felt comfortable using her in serious matches. However, they managed to fix two of her biggest issues with just one move. A recovery move that actually has good kill potential. ZSS's issues in Brawl heavily lied in her inability to recover reliably and to kill consistently. Her new up special really helps to alleviate both of these issues, and she still has the same great combo game from Brawl. What's even better is that ZSS can use her down smash to set up the up special when you want to use it for a solid, easy kill. ZSS is still a bit light, and her forward smash is really about the only other decent kill move she has (needs to be sweetspotted) but the combo game is amazing, and she finally has reliable recovery ability.

Lucario: Lucario was my 2nd main in Brawl, and thus far, he continues to be that for me in 3DS. Luke's moves have largely stayed the same, but his Aura is much more effective this time around. It scales much faster, and isn't effected negatively if Lucario is ahead in a stock match (it was in Brawl) Aside from his Aura, his attacks haven't really changed in concept. Force Palm now has longer reach the more damage Lucario has taken while ExtremeSpeed has amazing recovery range when Lucario's damage is high, so much so that you'll almost always have to offscreen Luke to KO him. Luke still has a really tough time getting KOs at lower percentages, but the faster Aura scale has helped to alleviate this issue a bit. Still a bit floaty and Double Team feels slower than in Brawl, but the buffs outweigh the small nerfs.

Lucina: This was a pretty big shock to me. For those who aren't familiar with me in Smash (probably all of you) Marth was my #4 in Melee and #3 in Brawl. When Lucina was announced and was said to have her power spread throughout the sword instead of on the tip, I was a bit skeptical on how well she would play. I have no idea why I was skeptical. Lucina's been nothing but impressive for me. Aside from the terrible recovery option and slightly lower kill potential, Lucina's ability to not have to get the tip for maximum knockback is amazing. Forward smash is a great kill move and most of her attacks feel marginally faster than Marth's, especially her air attacks. Early on, Lucina clocks in at #3 for me. While I'm not sure how long that will last as I try to get used to the changes to Falco and Marth, I'm pretty sure she will continue to be a character I use consistently.

Falco: My main in Melee and Brawl comes in at #4 early on. I feel my main issue is getting used to the fact that his Blaster isn't nearly as good and that his Reflector isn't a good offensive spacing weapon anymore. His air movements feel a bit more sluggish, but I already feel myself getting more used to that. His tail slash (down tilt) feels a little bit stronger than it was in Brawl, but still not as strong as in Melee. Up smash is a double hit now to separate it more from Fox (who's much improved I feel and almost made my early top 5) Down air is much weaker than in previous games, but I never really used it much in previous games until my opponent was at high damage, so that probably won't affect me with him much. Being able to act out of the side special is a great asset for recovering, which had previously been a major issue for Falco in past games. He's still got solid combos to work with, and all of his attacks are still pretty quick. Air attacks still have pretty low lag, so it's easy to get quick hits in. Recovery buffed, but nerfs to down air, Blaster, and Reflector gives Falco an early mixed bag for me. Still good enough for #4, but idrk how long that will last as I get used to other characters. Who knows, maybe I'll adjust quickly enough to main him again.

Shulk: Shulk was one of the characters I was most looking forward to using when the game came out, just to see how the Monado worked. While the powers may take some getting used to, they're well worth the time. Being able to switch up your own stats in battle depending on the situation is amazing. Shulk definitely has issues with getting the spacing needed to switch freely between powers, but they aren't totally central to everything. This can let you get one quickly, attack to get the opponent away, and then pick the one you really want. Buster is a fantastic damage dealing power with less knockback. Smash is the opposite and a great follow-up if you can nail down some power hits with Buster. Speed gives Shulk good combo ability, and also allows him to get out of tight situations. Shield is pretty self-explanitory, but I find it to be the least useful since it makes Shulk incredibly sluggish. Then there's Jump. Jump can work for defense and offense, probably being the most balanced of all the powers. Jump gives Shulk amazing recovery range, but can also give him amazing edgeguarding range as well. Just don't let Kirby get this power. Aside from the powers, Shulk has issues with recovery if he doesn't have the Jump power activated since his normal recovery is lacking severely. Otherwise, the Monado gives Shulk great range with most of his attacks and his counter move (down special) is absolutely devastating if you succeed with it, capable of KOing characters using smash attacks at around 90% (work's like Marth's, Ike's, and Lucina's in terms of damage / knockback) Really look forward to using Shulk more to see if I can get even better with him.

Honorable Mentions: I'm not good enough with these characters to put them in top 5 yet, but they're showing solid potential to me thus far and I plan on learning them a bit more after I try out every character:

Mario, Robin, Fox, Mii Brawler, Marth, Greninja, and Mega Man