if we consider ethash based coins then rx 5xx series is the best for it
in second hand you can get it easily for around 100usd

cryptonight based coins like xmr and others have migrated to randomx so it is useless to talk about because it is cpu area now

other coins like raven grin and others are an expertise of nvidia gpus
getting high end last gen gpus like 1080 or 1080ti second hand will provide you good value and double the hashrate for half the space
also get zotac as they have 5y warranty which is pretty sweet