For £50 the cards are "greatest" at a particular thing. One may be better at driving multiple displays, one may have passive cooling and be best for a quiet home theatre system and one may be clocked higher and better for gaming and be fan cooled.
It strongly depends what you want the card to do in this price range.
Used cards sub £60 on ebay in order of performance:
AMD/ATI Radeon HD 6870
AMD/ATI Radeon 5850
Nvidia GTX470 & GTX570
AMD Radeon 6770
AMD/ATI Radeon HD 5670 (Still OK, but avoid on Windows 10. Good for low power use.)
AMD/ATI Radeon HD 4890 (last resort)
None of these cards are going to be any match for a GTX 1080ti, but they are MUCH better than new cards at the same price (IE the GT710 and R5 240, which are utterly useless). Just watch out for the power use on these cards (They will also run much hotter)
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