The theme music.
Love Squall, the 2nd ending theme for the Red Jacket anime, is soothing to the ears.
Then there's Funny Walk in Old Fashion. Lupin III's soundtrack is fantastic overall.
. Tsuki to Taiyou no Meguri Aya Hisakawa, you're a goddess.
Yuji Ohno's soundtrack in general. He is the main composer, for the franchise, after all.
Zenigata March. Also has an epic lyrical version.
Then there's the opening of the more overlooked Pink Jacket anime, Sexy Adventure.
Lupin the Fire by SEAMO.
Memory Of Smile doubles as Heartwarming Music. The lyrics tell of a chance encounter and missing the smile on her face. What's more? It's sung by Lupin's voice actor!
Distant Winds from also by Lupin III: The Secret of Twilight Gemini Aya Hisakawa. A song so ambient, it can put you in a trance.
Duty Friend, the ending theme to the 2012 series, , is pretty freaking jazzy and catchy.
Lupin III: The Woman Called Fujiko Mine For the Lupin the III Cover/Remix Album, MONKEY MAJIK did an oh-so sweet rendition of the iconic theme song.
The first Italian opening, Planet O.