Hello, I'm looking for, besides class based games like Octopath Traveler, a game where you need to create your party by default.

Something like Icewind Dale 1+2 and Storm of Zehir.

Now I played:
Pathfinder Kingmaker
Tower of Time
Pillars of Eternity 1+2
All D&D classic RPG games
Divinity series
Expeditions Vikings
Bard's Tale IV
Octopath Traveler - it counts since it's class based, party based
Regalia oMaM - hmmm, maybe?
For the King
Thea 1 and 2 - maybe?

I will not play Numenera

Game must be party based and have true class based system. During my play of Divinity OS2, I don't find anything about it to be class based. It doesn't have to be old and it doesn't have to be anything like D&D.

But main thing is that you can choose your own party in game you can recommend.

Lords of Xulima (one mandatory character, rest created)
Might and Magic series
Serpent in the Staglands (one mandatory but customizable character, for the rest you can choose to create or look for companions in the world, or mix and match)

- Knights of the Chalice
- Avernum series (not 100% sure but I think those let you create a party while in Geneforge and Avadon you start with one player character only?)
- Legend of Grimrock 1&2
- Dungeon Master / Return to Chaos (although you don't exactly create the party members, you choose them)
- Might & Magic 1-X (as already suggested by Cavalary, I see)
- Realms of Arkania 1-3 (as well as the indie remakes)

You wrote all D&D classic RPG games, so I assume you've already played Temple of Elemental Evil? But did you also play the old Goldbox games (Forgotten Realms - The Archives - Collection 2)? I especially recommend Forgotten Realms Unlimited Adventures, if not. There are also the Dark Sun games and the Krynn series. In all of those you create your own party.

Dungeon Master!

The Wizardry series, you can spend hours just creating characters.

Wizardry 6, 7, and 8 are available here on GOG.

But not MM8 (you only create one character here).

But not Wizardry 4 (not available on GOG), where you don't create anyone (you play as the evil mage Werdna); then again, that particular game is not recommended for anyone who hasn't played at least Wizardry 1. (Start with 1, 5, 6, 7, or 8; 2-3 require imported parties, and 4 is not for beginners.)

Some other games on GOG with party creation and classes:
Ultima 3 (but not the rest of the series)
Bard's Tale 1-3 (including the remastered versions)
Elminage Gothic
(from what I hear) Mary Skelter: Nightmares

Some JRPGs not on GOG with party creation:
Dragon Quest 3 and 9 (3 has one mandatory character, but you can choose their gender)
Final Fantasy 1 (3 and 5 don't have party creation, but do let you choose classes and change them whenever you want)
SaGa 1 and 2 (released in the US as Final Fantasy Legend 1 and 2) doesn't have traditional fantasy classes, but it does have races that are fundamentally different from each other (as in, the very rules for character growth differ from race to race; also, different races are good at different things (in SaGa 1, humans can't effectively use magic, but espers can, for example))
Etrian Odyssey series

if Octopath Traveler counts, then we can count Final Fantasy 3 and 5 as well. Final Fantasy 4 is iffy (each character has a specific class, which can't be changed, and your party changes as the story progresses), and 2 is probably out (classless character growth system). Could also probably count games like that one Nintendo DS Final Fantasy: 4 Heroes of Light (something like that), and if you have a 3DS, perhaos Bravely Default/Second (also Final Fantasy Dimensions for smartphones). That's correct.

Geneforge has you play as one character, but you can use persistent summons if you need more (and your character can cast any).
Avadon you start with one character and recruit others, much like in Baldur's Gate. Doesn't really have what I would call a class system in the usual sense.

(Emphasis mine) She wasn't looking for class based games originally.

As for Thea I would say you can't actually create every party member from a scratch, but instead you can choose randomly generated characters.

I can recommend Galaxy Squad. You can chose the starting party there based on characters classes and even customize them. The game is similar to XCOM though.

Speaking of which, can't you create your own party in XCOM: Enemy Unknown as well? Do the characters have classes?

Also, Knights of Pen and Paper +1 & 2 and Galaxy of Pen and Paper..

And Darklands, although I'm not sure to what degree it has "classes" (characters seem to have "backgrounds" and "occupations").

In Dragon's Dogma (yes, it always makes an appearance in your threads ;) ), you can only create one character but later on choose from characters that other people created in order to form a party, and IIRC, you can level up and build them to your liking afterwards, too. Classes were rather fluid though, I think, not strictly defined and unchangeable.

You didn't stipulate if you wanted turn-based gameplay.
The TBS UFO games have random characters that you can hire, upgrade (with experience) and fire (if you don't like them):
— Aftermath (runs without ShaderModel 2, which is part of DirectX 9.0c, so it will work without a dedicated graphics card, if that is a limitation); and
— Aftershock, I have tried.
There are sequels, too. :)