These are based off my opinions and research. A Pokemon is more likely to be on these lists if their first type is the listed type. Flying-types have the most exception here, and my list revolves around actual birds. Not very many legendaries will be in this, as their uber stats already make them soar above most Pokemon.
Scyther-It has high attack power and a good speed. I reccomend teaching it X-scissor, or Double Hit if your Scyther has Technician. Scyther has also always been one of my favorite Pokemon. Scyther can be a demon if it has Double Team.
Heracross-Having sturdy stats and strong attacks, Heracross is one of the best Bug types. Be sure to teach it Stone Edge to trump those pesky Flying-type enemies. Reversal is also widely used for this Pokemon.
Armaldo-Armaldo also has a good type combination. Its defenses are more than decent, thanks to its Rock type, and it also has high attack power. Give it balanced attacks, as well as something like Toxic.
Yanmega-Yanmega has well-rounded stats, and can learn pretty good attcks. I would reccomend Bug Buzz and Air Slash, because Yanmega has higher Special Attack then Attack.
Houndoom-Houndoom's high Special Attack is extremely useful, just teach it Flamethrower and Dark Pulse, and let it conquer and destroy your enemies. Thunder Fang may also be a wise option, to defeat water types.
Tyranitar-With massive amounts of Attack power, Tyranitar can be a deadly powerhouse. If you're scared of Fighting types, Aerial Ace should do the trick. Tyranitar is also good for defending most attacks. A Quick Claw would do him/her good.
Absol-Absol can be used as a critical hit machine if it has the Super Luck ability. Give it a Scope Lens, and teach it a few high-critical hit ratio moves. It can learn Psycho Cut, Night Slash, and Slash. With this, use Swords Dance, and just attack. Your moves will have three times the critical hit ratio.
Honchkrow-An all-around good attacker, Honchkrow can also have the Super Luck ability. It can also learn Night Slash. Use its naturally learned attacks to your advantage.
Dragonite-Having a quadruple weakness to Ice type attacks, teach the sturdy, well-rounded attacker a Fire type attack. Electric attacks can also work for this, as many Ice-types are also Water. Dragonite has a huge array of attacks it can learn.
Flygon-Being faster then its rival Salamance, Flygon is an under rated, useful Dragon type. Don't forget to teach it a Fire attack as well, because of its quadruple weakness to ice.
Salamance-Salamance can withstand a few more attacks then Flygon, but it is slower. Salamance has a huge Attack stat, so Fire Fang should do the trick for its fire attack. Basically, all dragons should know a Fire-type attack except Palkia and Kingdra.
Latias-Latias is a really good Pokemon to teach Calm Mind. Just power up to near invincibleness, and Dragon Pulse away. Recover is also a good option.
Electrode-An insane amount of speed can really help Electrode, whether you want to Thunder Wave all your enemies, or wipe out Water and Flying enemies that won't know what hit them. A highly underated Pokemon in my end of the woods.
Luxray-This is probably a bit of a no-brainer, as many trainers have Luxray. Its ability to learn Dark-type attacks help it, so Crunch and Thunder Fang should be staples of this high attack powered Pokemon's moveset.
Magnezone-Magnezone has great defenses, and is very useful. I didn't realize their power until I trained one myself. Watch out for Ground types, however, because Magnezone doesn't have the Levitate ability. Magnet rise could become useful.
Rotom-Thanks to its Levitate ability, the rare Rotom is only weak to Ghost and Dark attacks. Rotom can also learn some spiffy attacks and is good at status-inducing.
Machamp-Use Machamp's ability(either Guts or No Guard)to your advantage. Facade is a good attack option if your Machamp has Guts, and Dynamic Punch if it has No Guard. Machamp can be a bit risky this way, so be sure to take care.
Hitmontop-Hitmontop has some of the best fighting type attacks in its movepool, and also a high Special Defense stat. Definetly an underrated fighter.
Breloom-Outfit Breloom with an item to fit its nature, either Effect Spore or Toxic Heal. Breloom learns a wide variety of useful moves, so outfit it to your own battling style. Stone Edge is a must if you don't want to be obliterated by Flying-Types.
Gallade-This may be because of my love for this Psychic-Fighting Pokemon, but Gallade is awesome. He can learn a wide variety of high-critical ratio moves, along with a high Attack stat.
Charizard-Quite a no-brainer, Charizard has been long-loved for its high Special Attack and massive pool of non-STAB egg moves. An extremely diverse Charizard will have attacks such as Earthquake, Thunderpunch, Ominous Wind, Crunch, Dragon Rush, or Outrage. Fit your Charizrd to your style.
Typlosion-Learning many of the best Fire attacks, Typhlosion is a sure-way to go for plain Fire-types. Charcoal and Blaze increase its firepower even more. Crush and burn!
Camerupt-Solid Rock is a surefire ability for Camerupt, softening up deadly Water attacks. Powerful attack, both Special and Physical should be in its moveset, because both are spectacular stats to this sturdy sweeper.
Infernape-A more balanced Blaziken, Infernape is another great Fire type with high Attack and Special Attack to back it.
Fearow-Higher Attack then Pidgeot makes Fearow a speedy Physical sweeper, though slightly more fragile. Guard Fearow with your partner if you want to risk a Sky Attack. Fearow goes wonderfully with Pokemon with the Lightingrod ability.
Noctowl-Though it misses out on Psychic STABS, Noctowl has more balanced and specialized stats than Xatu. High HP and Special Defense makes Noctowl lethal with Roost, Psychic, Calm Mind, and Air Slash.
Skarmory-Skarmory has great defenses that leave it more advantages over the normal birdies. Its attacks are just as varied, Bulk Up may frustrate your enemy to insanity.
Staraptor-Staraptor has marvelous Attack power, making its Close Combat a threat to many enemies.