Most 3* and below servants honestly, since my few SR’s(9 pulls+ starter and event servants) and SSR’s (4 pulls) are actually spread out and cover most of my needs without having to go to the lower rarities.
A few special cases of selective forgetfulness though:
1)I always forget that Ushiwakamaru is a 3* Rider because I grailed mine to 80. So whenever I pull a silver copy of her from the gacha I always go: “Oh yeah, she’s a Rare”.
2)When I think about my servants in terms of class, I always forget that Nero is a Saber. Even though she’s my second favorite waifu after Tamamo. Even when I pulled her (During Nerofest no less, so I knew she was on rate-up), when I saw the gold saber card I thought I had gotten Lancelot. Then I saw her, thought “Oh, yeah, she’s a Saber” and then was even happier than if I had gotten him.
3)Martha is actually an AoE Rider. Since I only leveled her up for the Pillar farming in the Temple, and I just used her skills before switching her out, so I never actually even attacked with her, I always forget that Santa Alter isn’t my only AoE Rider.