Power Level: Though we cant really judge power level anymore due to the recent releases of Dragon Ball Z movies ( Battle of the gods and Resurrection F). I can estimated Piccolos power level is around 550,000,000 (at the end of the Buu saga), which is is strong enough to go toe to toe with Cell. Now, power level is a bad thing to base this fight on given it can be immeasurable.
Fighting Style/Technique: Piccolo uses Ki as source of energy. Ki can be used in many different ways such as blasts, power boosts, and other things. Piccolo has mastered his Ki energy over a long period of time giving him great fighting ability and immense strength. Buku-Jutsu (literally means Air-Dance) is what fighting style Piccolo uses, Piccolo likes the air and will take the fight to the when needed the Buku-Jutsu is the ability that give Piccolo flight. He is skilled in martial arts and uses special Ki blasts. What Makes Piccolo So Dangerous? His skill in martial arts and deadly Ki blast mixed into on fighting and flight makes for one tough opponent.
Signature Moves: Piccolo has a lot of moves coming from the Ki source.
Antenna Beam- beam from antennas
Basic Ki blast- a shot of Ki from his hand
Explosive Demon Wave- Very powerful energy blast shot from one hand.
Hellzone Grenade- Scattered shot of blasts surrounding an opponent and are guided to blast the opponent creating an explosion
Instant Transmission- Can instantly teleport any where but need to have focus on where.
And probably the most famous of them all is....
Special Beam Cannon- Most powerful attack in his arsenal, drills through anything and every being, but is very time consuming and needs about id say 5-7 minutes to charge, or if this was DBZ universe about 3 episodes.
Personality/Moral: Piccolo is often grumpy and cold hearted. He likes to be alone rather than with people. Piccolo was once a villain in Dragon ball but made the change to good. With Piccolo once a villain, he posses villain like qualities which can result in ruthless fighting and no mercy type style. This villain like condition can be used in battle resulting in anger and potentially give a power boost. Piccolos moral isn't just to fight good when evil is on his door step, he is more of a vigilante when it comes to this. Piccolo wants to eliminate the threat before it gets stronger, smarter, faster, and anything else. Piccolo wont let his enemy reach full potential like Goku and Vegeta, He would rather destroy them quickly. Piccolo is not afraid to kill in order to save planet earth. He knows what destruction can be cause buy some of the foes he has faced. Given time and knowledge of an opponent Piccolo can train in the hyperbolic time chamber, which lets him train for a whole year in a few days. This can increase his power substantially and he could ultimately defeat his opponent.
Armor/Clothing: Piccolo does not wear armor but wears weighted clothes to mask some of his power, these clothes (shown above) can slow him down in a fight. When removed power level and speed increase, but given a foe like Green Lantern who doesn't understand power level this is really useless.
Incredible Feats: Piccolo has fought some of the toughest villains in DBZ. He doesn't go down without a fight either he has incredible will and lets face it can take one hell of a beating. He killed Raditz and Goku too, if you wanna count that. Has gone toe to toe with some of the best, and has trained Gohan, Trunks, And Goten who are some of the most powerful characters in DBZ. Has survived in space and under water for long periods of time.
Hal Jordan is the most popular so I will be using him.
Power Level: Power level is not something this fight should be based on but I had to talk about it anyway. Green Lanterns uses a ring that uses pure willpower to power the rings abilities. Now if you remove the ring he is powerless, but how much will does one person have? You could say will is infinite, if will is infinite then you can say that Green Lantern has unlimited power. If he has unlimited power then he is one hard man to stop. Downside, the ring must be charged every 24 hours to continue to give Hal Jordan his powers.
Fighting Style/Technique: Green Lanterns use the green ring as there source of fighting being able to create anything that you imagine by solid green light can be very lethal. This ring also gives him flight and ability to breath in space. Now creating objects with his ring are his only way of fighting. Hal has mastered the power of the ring giving him full control of its abilities. Hal Jordan was in the air force, which means he must know the ability of SOCP the fighting style of the U.S army rangers. He might not know the exact style but since he is of military background I will say that is a factor. Combine the SOCP with the ring and you could have some deadly combos. He also prefers air combat rather than on the ground making this a solid fight for Piccolo.
Signature Moves: The Ring is more than just creating stuff.
Mind Control- Hal can use the ring for mind control, but it takes a serious toll on his ring.
Energy Projection Blast- A blast from the energy of his ring.
A.I- The ring is connected to the main battery on OA. This battery acts as an on-board computer give Hal knowledge of all things he wants to know.
Animating- Ability to create anything he can imagine.
Wormhole- Creates a wormhole.
Illusions- Can Create an illusion for the opponent.
and if you didn't know....
Invisibility- Lets the user become invisible but requires strong concentration.
Personality/ Moral: Hal is the good guy, the one who is going to fight evil every time. Hal is known to be a little bit arrogant, not to the extent where it ruins his way of thinking. Hal would much rather have his villain locked up rather than killed but has defeated and killed many villains. As a kid Hal Jordan had seen his father die in a plane crash, despite that he has stayed pretty mentally stable, but is prone for "flashbacks" cause some mental distress. Given that his ring runs on pure willpower, Hal has a very good amount of will and would never back down from a fight, this makes him a difficult opponent to defeat. Given time Hal can defeat any opponent as long as his will is still there. Green Lantern can be a tough opponent as long as he stays focus and the will energy is still intact.
Armor/ Clothing: Green lanterns clothing is his basic green and black suit. He doesn't wear any armor although his ring has the ability to let him wear whatever he wants if that means full body armor then full body armor.
Incredible Feats: Hal Jordan has done some incredible feats, some of these feats include saving the earth and other planets multiple times. Hal is one of the founding members of the Justice League and is one of the most powerful heroes in it. He has over come the color yellow or fear becoming one of the only Green Lanterns to do so. Yellow/ fear is one of the only weaknesses that the Green Lanterns have. Hal has also stepped up to fight members of the Justice League in order to keep them focused and on task. Often known as one of the better fighters of the JL( Justice League) . You cannot forget about the fact that he has knowledge about the entire universe. Leadership is another trait Hal has and has lead Justice League attacks. All of these feats make him a tough opponent.
Why? One, Dragonball Z characters are on just a different level than regular superheroes. The combination of martial arts skill and energy blasts make to much of a deadly combo. Don't give me wrong Green Lantern would put up a hell of a fight from the air, but Piccolo would exploit that weakness and take it close quarters, and that is where he would win it. Add that with Piccolos ability to get messed up in a fight and you have a hard guy to beat.