A wild card plays as whatever rank would make the lowest hand. Thus, in 6-5-Joker-2-A, the joker plays as a 3, while in Joker-5-4-3-2 it would play as a 7 (an ace or six would make a straight).
a wiLd card PLaYs as whATEver rAnK would MaKe tHE LowesT HaNd. ThUS, iN 6-5-JoKEr-2-a, tHe joker plays As a 3, WHile In JokeR-5-4-3-2 IT would pLay As a 7 (an aCe or sIx woUld Make a sTrAIght).