Suit:
A two piece matched suit is always the best and safest choice.
Conservative colors / fabric:
Navy, dark gray (and black for women) are safe.
Other color trends may come and go; avoid the extremes.
Solids or very subtle weave patterns or plaids (the kind that look solid across a room) are safest.
Wool, wool blends, or good quality micro fiber for women only, are generally the best fabrics in all seasons. Avoid acetate / rayon blends.