Size and popularity don’t necessarily translate to quality or value for money. I’d try to get a personal recommendation from someone you know and trust. Avoid any flat monthly fee offers (the less the agency invests, the more profit for the agency). Avoid any agency not offering to report each and every link with its respective cost. Avoid any agency instantly sending you a huge portfolio to pick from, these lists will be everywhere, including the hands of the Google webspam team.