There's unlikely will be one firm which is always 'cheapest'. Company A might be cheaper than company B for hatchbacks but dearer for 4x4's. Company C might be cheapest for 'return to base' hire, while company D will be better for 'leave it anywhere' deals.
The only way to find out which firm is best for you is to phone them up (or get online quotes) to see what they can offer. Contact details are here:
http://www.belfastairport.com/en/content.asp?a rea=2&id=186
Chris