1. Sheik is the best character in the game. She's really safe and really strong in the neutral, amazing ground and air game, approach, recovery and out of shield options. She can get really ridiculous punishes if you make even the tiniest of mistakes. She also has medium-lightweight and average falling speed.
2. Ness is better than ZSS. His downB/side smash nullifies ZSS's neutralB and if you're fast, you can avoid the "bait" she had set (luring you to absorb the projectile then grab) and get a free grab, potentially leading to a 0-death combo.
3. Luigi and Pikachu is even. It might be slightly in Pikachu's favor because he has a lot of options to gimp Luigi, but if you're good with Luigi's downB mashing, then it's 50-50.
4. Diddy in my opinion should be higher. It's not like his nerf made him bad or something, as lately ZeRo had proven to us all at EVO that Diddy is still highly viable. He might be better than Ness, ZSS, or Sonic, but since patch 1.0.6, he begin losing representation in tournaments.
5. I think Marth is still viable. Yes, I'm making an assumption. It's not like I'm biased and think it that way just because he's my main, as he's getting buffed pretty much each patch and I think will catch up pretty soon to the mid or the upper mid tier.
6. Ike can potentially be an upper mid tier. His buffs are significant, and if this keeps up he would be rocketing a few tiers. He might be even better than Marth. His tournament representation however, is usually limited to MVG Ryo, which is known as the best Ike in the world and had gotten okay results. If he gets more players then he would be higher, I believe.
7. Reminder that this is my opinion. Keep in mind that the metagame of Smash 4 is widely in development. You're welcomed to disagree with this, but as always, I'm just giving you about what I think. Thank you for reading as always, have a good one.