Depends on what for. The best type combo for a defensive Pokemon is way different from the best type combo for an offensive Pokemon, and the worst type combo for a defensive Pokemon is way different from the worst type combo for an offensive Pokemon.
Best Offensive: Probably Ghost/Fighting, maybe give it Scrappy in case a Ghost/Normal ever shows up.
Worst Offensive: Pure Normal. It's just terrible.
Best Defense: Probably Fairy/Steel with Levitate. Only one weakness and several great resistances, including poison immunity.
Worst Defensive: Probably Rock/Ice. Though, this is actually pretty hard to pin down because of how defensive cores can make seemingly bad defensive types shine given the right resistances, like how Celebi complements Heatran.
Good Offensive typings will typically include Rock, Ice, Fighting, Ground, and sometimes Dragon in them. Water, Dark and Ghost aren't too shabby either. Electric is only good because Water is a great defensive type.
Good Defensive typings will typically include Steel, Water, Dragon, and Ground while avoiding types such as Rock or Ice. --- "Religion keeps pressing B when we try to evolve." - pirategeorge" [3DS FC: 1934-0673-9848 (Pentao)]