Eharmony and Match.com are by far the most reviewed sites, according to Consumer Affairs. Match.com has over 1.8 million subscribers, as of November 2015, meaning that users can usually find plenty of potential matches in their areas. Since its a paid site, most active users are typically serious about dating. EHarmony also has a large user pool and is strongly focused on helping people find serious relationships.
Sources: consumeraffairs.com, marieclaire.co.uk, ebizmba.com