At that voltage level, I presume these cables are going to be single conductor.  As a general rule the armour on single conductor cables should be non-ferrous ie. aluminium.  At 66 kV, I would check with the manufacturers (Pirelli, BICC, Okonite, etc.) for this application.

Suggestion: Provide more information, e.g.
1. Application
2. Cost importance
3. Physical injury
4. AC or DC
5. Underground or in the air/tray
6. Etc. whatever else you think that may be important factor

I found some information I had from Pirelli for 25 kV cables - generally applicable to 66 kV as well I presume.

From Pirelli:
"Available armour types:
1. Interlocking armour of aluminum or galvanized steel.
2. Galvanized steel wire armour.  On single conductor cable, this type of armour is available per ICEA S-66-524 Section 4.
3. Double steel tape armour (DSTA).  This armour consists of two layers of gapped galvanized steel tape applied over the inner jacket followed by an outer jacket.

Note: Steel interlocking and steel tape armours are not recommended for single conductor cables due to the excessive magnetic losses."

I would stick with the interlocking aluminum armour unless these cables are to be vertically suspended for some distance, in which case the steel wire armour would be preferred for supporting the cable.