Class the most similar to Alienist

[rOnzO the gOblin]

I'm looking for a class who match the most possible with the alienist of D&D3.5

[Milo v3]

I'd say Summoner. Describe the creatures you summon monster as being horrific in appearance (fiendish templated monsters are much easier to accomplish this with than celestial templated monsters). Have your eidolon be eldritch and wrong.

[Cycada]

Morphic Savant Unchained Summoner is exactly what you want.

Essentially a spellcaster who harnesses chaos-infused summoned beings that change form on a whim. The personal trauma of doing stuff like this (as represented by the insanity-related stuff of the Alienist in 3.5) will have to be left up to traits/flaws and general roleplay. I feel this is the closest you'll get to something that thematic, though.

[I'm Hiding In Your Closet]

Another option is an Aberrant-Blooded Sorcerer who specializes in Conjuration - and the Monster Summoner's Handbook has some more good things to get your summoned monsters even closer to what you're aiming for.

Also look at some of the expanded summon monster list from various older sources.

Then of course there's this little wonder, which I have personally playtested and left notes on.

[born_of_fire]

The dark tapestry mystery for oracles is thematically appropriate IMHO. Find a suitable curse, focus on summoning spells and Bob's your uncle.

[I'm Hiding In Your Closet]

The issue of arcane vs divine may matter there, though - I guess that's OP's call.

[born_of_fire]

That's a good point. Spirit guide oracles can gain access to arcane spells if that helps at all. Not sure what aspect of Alienist OP most wants to capture.

[I'm Hiding In Your Closet]

Elven Lorekeepers, too.

By the way, thanks for saying that was a good point - I've become overly used to the discourse around here shifting to a more course and shallow level of thinking that dismissively shears off the bulk of the game as if it doesn't mean anything.