Man you chewed through Alita fast. Did you enjoy it?
For stuff with a similar vibe I would recommend (off the top of my head):
Ashen Victor (a motorball spinoff short by Yukito Kishiro, set before the events of alita)
Appleseed (the mech design aesthetic here has influenced me a lot)
Akira (obviously)
Ghost in the Shell (also obviously)
Biomega
BLAME (this one and Biomega are very similar and by the same author, very grungy)
Gantz (a very.. strange series. think men in black, just soul crushing nihilism edition)
Zetman (cyberpunk superhero origin story with tons of body horror and great art)
For more unrelated manga I'd still recommend to anyone who is suave enough to post on this forum:
Planetes (one of very few realistic sci-fi manga out there)
Dorohedoro (an amnesiac guy with a lizard face eats the heads of magicians so they can talk to the man inside his throat. yes, that is all you need to know)
Berserk (the classic, numbingly grim yet exhileratingly vibrant fantasy revenge story that will never ever be completed, so be warned that you're entering a contract with the devil when you start reading it)
Shingeki no Kyojin (giants who mindlessly consume human beings wander the earth, trapping humanity in a single walled country. characters who use their brains, horrifying monsters, unpredictable plot. good stuff)
Vinland Saga (vikings. without horned helmets. with historical characters. and great art)
Teppuu (arrogant girl gets into MMA. no metaphysical ***, no cheesy plot points, just really good fighting)
Goodnight Pun Pun (a little kid grows up to be a profoundly broken human being, rendered in a touching and humorous way with a metaphorical art style that rarely shows how the main character actually appears)
GYO and Uzumaki (big horror classics)
Hoshi no Samidare (an enormous hammer is poised to smash the *** out of the earth. a lizard appears in a guy's bedroom and offers him the chance to be a hero)
Aqua Knight (series that Yukito Kishiro started between finishing Alita and starting Last Order. only two volumes iirc but quite interesting)
Homunculus (weird psychological horror about a guy with profound mental issues who gets a heightened sense of empathy after receiving trepanation)
Yotsuba& (amazingly charming slice-of-life series about a single dad and his energetic kid and their neighbors)
Franken Fran (frankenstein chick helps her "patients" cope with their problems.. in horrible ways)
Bakuman (shounen about writing manga. by the Death Note guys)
One-Punch Man (a guy wants to become a hero! too bad he destroys every enemy with one punch, making his life unbearably boring. utterly amazing artwork, possibly some of the best in the entire business, and *** on every single convention of shonen manga)
Solanin (a fantastic manga about the apathy of mid-20-year old life)
Shamo (a kid kills his parents in cold blood and gets sent to juvenile prison, learns karate from a convict and becomes the biggest monster in his new world)