My favorite is G-Mantle. Then probably Kouryu, the insane version of Gaisei. He's got that kind of Walter E. Kurtz/Dock Tobin vibe attached to him.
Honorable mentions:
Roddy & Cathy had huge potential, but it remained underdeveloped. Just like Karman Cole's, who is my favorite German character
.
The Cyber-Lip supercomputer is great, too. It gives Rick and Brook, your standard military vets, an unexpected depth.
Ukyo is a classic. You don't see so many heroes stricken with tuberculosis, right? His animation while coughing blood is pure cult.
The late Ikki Kajiwara and Asao Takamori would surely argue about Joe being an SNK character.