Which His Tag Length, Position or Sequence to use?

Length and position of HisTags can affect the following:
Expression Yield
Efficiency of translation. Triplet codons located after ATG can inhibit translation. Use highest expressing AAA (Lysine) at position 2
Binding to IMAC resin
Results (activity, crystallization, NMR …etc). Remove the tag!
Oligomerization state

E.coli water channel AqpZ expression and purification using different His-Tags (Co = cobalt-IMAC, & subsequent Q2= ion-exchange)