I came across this tsumego while studying a (now defunct) joseki.
This is my first problem here so I welcome any comments/corrections/variations I might've missed.
(;AB[pd]AB[qf]AB[pb]AB[qb]AB[oc]AB[nb]AB[oa]AB[pf]AB[rg]AB[qh]AB[oe]AB[od]AW[rb]AW[qa]AW[qc]AW[pc]AW[qd]AW[qe]AW[pe]AW[rf]AW[se]C[White played tenuki and thought she was safe. Black to play the best move.]AP[goproblems]
(;B[ra];W[rc]
(;B[sg]
(;W[sa]
(;B[sc]C[RIGHT])
(;B[sf];W[re]C[CHOICE])
(;B[re]LB[sc:A]C[This is ko, but A killed.CHOICE])
(;B[sd];W[sc]C[CHOICE]))
(;W[sd];B[sb];W[sa]C[Ko is the best white can do.Technically not the best ko for white. Lee Sedol started at S16, which is the best ko - see comments.CHOICERIGHT]))
(;B[sb];W[sc])
(;B[rd];W[sa]C[CHOICE];B[sc];W[sd])
(;B[sd];W[sc]C[CHOICE])
(;B[sc];W[sd]C[CHOICE])
(;B[sa];W[rd]C[CHOICE])
(;B[pa];W[rd]C[CHOICE]))
(;B[sc];W[sb]
(;B[ra];W[rd]C[CHOICE])
(;B[rd];W[rc]C[CHOICE])
(;B[rc];W[rd]C[CHOICE])
(;B[sg]
(;W[rc]
(;B[ra];W[sd]C[CHOICE])
(;B[sf]
(;W[ra];B[re];W[sd]C[CHOICE])
(;W[re]LB[sd:a]LB[ra:b]C[CHOICE]))
(;B[re]C[CHOICE];W[rd]C[CHOICE]
(;B[ra];W[sf])
(;B[sf];W[ra];B[re];W[sd])))
(;W[sd]
(;B[re];W[rc];B[sf];W[ra]C[CHOICE])
(;B[ra];W[rc]C[CHOICE])
(;B[sf];W[rc]
(;B[re];W[ra]C[CHOICE])
(;B[ra];W[re]C[CHOICE]))))
(;B[sd];W[ra]C[CHOICE]))
(;B[sb];W[sc]
(;B[ra];W[sa]C[CHOICE])
(;B[rd];W[rc]C[CHOICE]))
(;B[rd];W[rc]
(;B[ra];W[sa]C[CHOICE];B[sc];W[sd])
(;B[sd];W[sc]C[CHOICE];B[ra];W[sa])
(;B[sc];W[sd]C[CHOICE])
(;B[sg]))
(;B[rc];W[rd]
(;B[ra];W[sb]C[CHOICE])
(;B[sb];W[sc]C[CHOICE];B[ra];W[sa])
(;B[sc];W[sb]C[CHOICE])
(;B[sg];W[sc]C[CHOICE]))
(;B[sg];W[sc]C[CHOICE]
(;B[rd];W[rc])
(;B[ra];W[rd];B[sf];W[re]C[CHOICE]))
(;B[sa];W[rd]C[CHOICE])
(;B[sd];W[sc]C[CHOICE])
(;B[pa];W[rd]
(;B[ra];W[sb]C[CHOICE])
(;B[sb];W[ra];B[sc];W[rc];B[sg];W[sf]C[CHOICE])
(;B[sc];W[rc]
(;B[ra];W[sb]C[CHOICE])
(;B[sg];W[sf]
(;B[ra];W[sb]C[CHOICE])
(;B[sb];W[ra]C[CHOICE]))
(;B[sb];W[ra];B[sg];W[sf]C[CHOICE]))
(;B[sg];W[sf]
(;B[sb];W[ra];B[sc];W[rc]C[CHOICE])
(;B[ra];W[sb]C[CHOICE]))))