Texhnolyze is a good choice
I often hear Ergo Proxy and Haibane Renmai mentioned in the same breath.
boogiepop phantom is pretty good
Despera's a pretty good one if you can find it.
I heard the light novel of boogiepop phantom is one of the greatest ever written, and the anime pales in comparison.
If you like more of the "I dont even understand whats going on" part check out Cat Soup.
What did you mean? The anime is dead.
Paranoia Agent comes to mind.
Haibane renmei can make a grown man cry, but it's nothing like lain
The true answer to OP's question>steinsgate
greeting from the future
picrelated, overreaction reaction image. i liked steins;gate, but have to kinda sorta disagree. yes and no. cyberpunk in it's way, but also a lot more comedy then lain. more moe. more intentionally self referential to anime as a genre unto itself. there is the john titor mythos and it's use in the story is similar to that one lain episode, layer:09, where there is the exposition on the various conspiracy theory type philosophizing that is a backdrop to sel. both riffing off of that building 90s conception of cyberpunk. where they really diverge for me is in the style and pacing. lain can be so slow, vague, and has such a weird mood to it. i've yet to see any other anime like lain that i could compare to it on those grounds. maybe if a director like david lynch directed an anime? the way sel story is told reminds me of something like a david lynch movie, where at the beginning you have no concrete idea about what is actually happening until the world that's being built explains itself somewhat and the film just seems like a vague dreamlike poem, with little narrative, made for the sake of the aesthetics. and even then, there is a lot left up in to interpretation. i do wish that i have seen more anime like that and could recommend something.
S;G and Lain are both 10/10 for me and I completely agree with you. Lain is more about transhumanism and the feeling is comparable to AKIRA or NGE. While S;G shines through the relatable characters and even though it is a /cy/-setting it's still a mixture of a lot of anime styles. Steins;Gate is a good story with brilliant characters told in the best way possible, but it doesn't really have such an actual message.