For me, the process of choosing a waifu is a lot of whether or not I actually like the story. If I can get invested enough in the story, then I'll start to absolutely love certain characters. If the story is bad or it's just not interesting, then no matter how cool certain characters are, it won't really matter.
Now when it comes to deciding the waifu of the series that I actually like, it's a lot about the character's personality and development. Looks rarely matter to me as much (although they certainly do help) because no matter how pretty a character looks, if their personality is boring and they're annoying, then I'm not gonna like them. Some examples of visually appealing designs but personalities I don't like include Chitose from Nisekoi, Uraraka from BNHA, Emilia from Re:Zero, and 02 from Darling in the FranXX. I specifically don't like tsunderes much.
I can't actually say what kind of personality appeals to me the most because I really don't have a specific one. I can say, however, a bunch of characters I do like. Rin Tohsaka (Fate/stay night), Saber (Fate/stay night), Caster (Fate/stay night), Mirai Kuriyama (Kyoukai no Kanata), Arcueid Brunestud (Tsukihime), Ram and Rem (Re:Zero), and Megumin (Konosuba).