First, the biggest bass horn in the world:
The most powerful:
Danley Matterhorn. This is a tapped horn with 40 high excursion drivers and 40kw of power. Designed for military use where 105db is required outdoors at 15 Hz one quarter of a kilometer away! The US military approached Tom Danley when they weren't able to meet their target with a sub that fits into a shipping container.
One of the more stylish options:
Avant Garde >
Now let's come back down to earth. Here are some designs that you could actually build and use in a domestic room.
William Cowan sub jr
Compact and large tapped horns
Here you will find a range of horns designed for pro use, as well as bandpass designs.
This is a diy favourite intended for pro use designed by Tom Danley. It uses the Eminence Lab 12 driver which was designed for the application. Some have had good results for home use.
Pro sound web info >
Pi Speakers bass horn kit
This is a commercial subwoofer that is based on the Lab horn - essentially an improved larger version.
Vincent Brient straight bass horn
30 Hz straight horn >