Tophat can be worn on mostly formal wear. Most kind of suit would fit with a tophat. The most commonly worn outfit suit with tophat is the morning suit. The tophat is also suited to tuxedo.
The morning suit has a morning coat (jacket), waistcoat and formal trousers. Morning suit is the traditional garment for men at weddings, formal memorial services, some official functions and formal daytime events.
The top hat is usually associated with the more formal attires. You can wear these hats with a range of outfits. However, all of them fall in the formal category. The most commonly worn outfit is the tuxedo with tails. This is the conventional outfit, often paired with the top hat. You can also pair gloves with the outfit to complete the look.
You can also try out the waistcoat style with a long 3 piece suit to complement the hat. Another interesting choice is the ascot suit which is not for the faint of heart. If you believe in strong fashion sense and making a statement, this is the suit for you.