Welcome to the Clinic, Hejin.
- Charizard's Nature does nothing but hurt his effectiveness since it cuts his Speed down and boosts his Attack, and the pros of this do not outweigh the cons. That aside, my personal favourite 'Zard set is the "Sub + 3 attacks" Special set since it turns Charizard's monstrous Stealth Rock weakness to his advantage by letting him activate Blaze very quickly, allowing for monstrously powerful Fire-type attacks. The three attacks you'd use for this are mostly personal preference - Flamethrower or Fire Blast are the best options for STAB, Substitute obviously takes a slot itself, and any two out of Dragon Pulse, Air Slash, Hidden Power (Electric or Grass are preferable, if you have either of those,) Focus Blast or Focus Punch for the rest of your set. Note that you'll want Timid or Modest nature for any set that doesn't use Focus Punch, with Hasty/Naive or Rash/Mild being the ones to pick if you do choose Focus Punch. Salac Berry is the best item for this.
- I've no idea what your Registeel is trying to accomplish here - it seems like you're attempting an offensive set with a Pokemon that has downright awful offensive stats and typing, and amazing defensive stats and typing. As such, Superpower can go right off the bat since it only serves to harm Registeel's defensive power really. Flash Cannon and Charge Beam can work, but Regice typically does this better thanks to his higher Sp.Attack and access to a far better STAB attack in Ice Beam. Amnesia can work out to your advantage since Registeel is immune to Toxic, but be warned that this may bait a PHazer in, who will Roar/Whirlwind you away for setting up and potentially drag a Pokemon out that you'd be better off saving - since PHazers most commonly show up in teams that use entry hazards (Stealth Rock,) this can turn quite disadvantageous.
- Gyarados's nature is beyond words awful since it drops its best stat - you'll want to change that nature to make the most of Gyarados. Besides that, your moveset and item are good. Earthquake and Stone Edge are potential alternative options to consider for Gyarados, and Taunt can work nicely too to lock down a more defensive Pokemon's moveset and allow you to set up Dragon Dance more easily.
- Yanmega, again, gains very little from his poor nature, but unlike Gyarados it at least doesn't harm his efficiency in any way. Wise Glasses or Life Orb are better options for hold items, although Life Orb can get Yanmega killed very quickly if Stealth Rock is on the field to slash his HP in half. AncientPower isn't a great coverage move for the dragonfly due to its low power really, moves like Hidden Power or Shadow Ball will likely serve him better. A good Hidden Power type, such as Fire, Fighting or Ground, would serve him best, and while AncientPower can work, the low power and very low PP make it a lackluster option really. Otherwise, Yanmega's good.
- If you can, get a Dragonite with ExtremeSpeed - priority moves are the big thing in this generation, and Dragonite's mediocre Speed is definitely far less of an issue with ExtremeSpeed on hand. Adamant tends not to be the best nature for him (although it is still very good) since it drops his Sp.Attack (while boosting his Attack) which lessens the effectiveness of mixed physical and special movesets, something Dragonite is usually better off aiming for. Consider Earthquake as a possible option for Dragonite, since it works fantastically alongside Outrage and Fire Punch.
- Drapion could do with ditching Dig since all it'll do is cause a Flying-type or levitator to switch in whilst you're underground, or even worse, prompt the opponent to hit you with an Earthquake (and since Drapion's weak to Ground, he will be a bit prone to having Earthquakes aimed at him.) Black Sludge is a better item for him than Toxic Plate since STAB Poison's coverage is terrible, and Drapion will likely be relying on Crunch for STAB more than Cross Poison anyway. Other moves to consider are Sucker Punch (this is by far the best option for Dark STAB on Drapion,) Toxic Spikes, X-Scissor or any of the elemental fang moves that Drapion has access to (I think he gets all three.)
Overall, as mentioned in chat, you currently have a massive Stealth Rock weakness that is pretty-much team crippling, especially since Stealth Rock is arguably the most commonly-seen move in the entire game competitively, and a shoe-in for best move in the game as well. Also, three of your Pokemon are weak to Electric-type attacks and no-one resists it, four Pokemon suffer weakness to Rock-type moves, and almost everything likely to carry a Rock-type move will carry a Ground or Fighting-type move along with it to aim at Drapion and Registeel. No Rapid Spin worsens the Stealth Rock problem, and you yourself lack Stealth Rock or any entry hazard at all for that matter. I'd consider all these factors mentioned and try to address them as best you can if you wish to improve.
