It's mostly the non-SSRs that get memed in FGO as trash-tier Servants. Among SRs there were Stheno, Siegfried, and Fionn. Among lower rarities there were Boudica, Geronimo, and Caster Gilles de Rais. Saber Gilles is a beast if used well, it's just that difficult to use him that well. Siegfried, Fionn, and Boudica were resurrected from the dead after their most recent upgrades. Stheno, however, remained trash-tier even after upgrades, while Geronimo did not get ANY upgrades at all.
But the one SSR that got memed for being trash-tier was Scheherazade. She got better after her upgrades, giving her niches in both CQs and NP looping against King-trait enemies, but before then she was considered very bad because she kept spooking people rolling for the Big Four Casters like Merlin and Skadi whenever they are on rate up.