Depends on what you are looking for. The most "touristy" area is El Poblado near Provenza. It has many nice bars, restaurants, clubs etc. Also many boutique style hotels that are very nice but offer mostly adults only rooms. Sabaneta is also nice but quieter and more suited for families.
Cocktail Attire: Wear cocktail dresses for women. Suits or jacket and slacks for men. Cooler temperatures in evening time due to mountain setting.