We also had a quite successful fighter who started off with some sort of chain mail and later swapped that for a sturdy yet quite manoeuvrable AT 18 - just as DS pointed out. Excellent piece of equipment.
For Rangers and most Semis I always liked AT 9 or 10 which are really good or, well, offer an excellent balance between protection, manoeuvrability, and spell casting penalties, except you are a essence spell-user, of course.