Best offensive: Ice/Electric. Nearly perfect coverage, tons of good STAB moves, as long as you are Special attacker. I physical, Ice/Ground or Ice Rock offers similarly good Physical coverage.
Best Defensive: Dark/Ghost. Only one weakness (Faeries), and three immunities (fighting, normal, psychic). Water/Steel makes a strong case too since it has lots of resistances.
Worst Offensive- Normal/Poison. Normal only really adds STAB return/Frustraion and a couple other moves to the table, and offers no super effective hits. Too many things resist poison, and Steel is immune to it. Also, there aren't a ton of high base power poison moves.
Worst Defensive- Ice/Bug. It's weak to, like, half the pokedex, gets shit on by common hazards, no immunities, x4 weak to rock and fire, dies to common priority moves like Bullet Punch and Brave Bird (might as well be a priority move in OU). It's only saving grace would be resisting Ground and Ice moves, but even then, ground types often carry rock attacks. Rock/Ice is pretty bad defensively too.
