Piccolo made his first appearance in the latter half of Dragon Ball (the series that preceded Dragon Ball Z, when Goku was just a boy.) He was introduced as the reincarnation of Demon King Piccolo.
The aforementioned King Piccolo was a powerful villain that once terrorised the Earth. With the efforts of Goku's mentor Master Roshi, Piccolo was sealed in an electric rice cooker using the Mafuba containment technique.
In a final desperate effort to survive, King Piccolo spit out an egg containing his reincarnated form -before promptly exploding. When Piccolo Jr. re-appeared at the World Martial Arts tournament, he was fully grown, intent on revenge and immediately became a formidable opponent for Goku.
What I've always liked about Piccolo is his brooding, cool guy attitude - he has a low tolerance for nonsense, particularly in the early sagas of Dragon Ball Z. He has consistently been the most powerful Z Warrior outside of the Saiyans and has even been the strongest on several occasions. He has had some pretty epic moments throughout the run of the show and that influenced why he has always been my favourite.
Stand out moments include when the Salyans arrive with their Saibamen minions that are roughly equivalent to Raditz, who Goku and Piccolo had such trouble defeating. Following on from the death of earthling Yamcha, one of the Saibamen attempts to attack Gohan. Piccolo grabs the creature by the arm, tosses it into the air and destroys it with a single mouth blast, literally stripping the flesh from its bones.
He rose to prominence again during the Frieza saga after having trained under King Kai and subsequently fusing with Nail, the Grand Elders protector. Following this, Piccolo became the strongest Z Warrior and was able to best Friezas 2d form. Sadly, he would be swiftly taken down by Frieze's 3rd form, and frankly humiliated by the Lord of the Universe.
Under similar circumstances during the Android saga, Piccolo fused with Kami - the God of Earth. King Piccolo had once been a single entity but split into a good and evil half so that Kami could take on the role. Upon the Androids arrival they decimated the Z Warriors, including the Salyans Trunks and Vegeta. Piccolo was once again able to cement his place as the strongest Z Warrior, surpassing even the Super Saiyan form that had been held in such high regard. Piccolo was able to fight on an equal footing with Android 17. who had earlier (alongside 18) decimated the Z fighters. Android 18, the weaker of the two, had earlier single handedly humiliated Vegeta in their fight. This fight is often regarded as one of the best in the series and concludes with 17 and Piccolo combining forces to take on Imperfect Cell.
Unfortunately. this was Piccolo's last truly great moment in Dragon Ball Z. His last few storylines mainly involved him looking after the kid Saiyans, teaching them the fusion dance and spending a period turned to stone by Supreme Demon King Dabur. Forfeiting his bout against the relatively weak Supreme Kai in the 25th Tenkaichi Budokai was also a low for the Namekian.
I am pleased to say that he has had a bit of a redemption of late, getting a new evolved form known as "Orange Piccolo" in the recent Dragon Ball Super Super Hero which has once again brought him up to a level rivalling the saiyans once again.