I'll opt for Comodo PositiveSSL Certificate, it is a single domain certificate and the main reason to choose this certificate is it offers speedy insurance and 256-bit encryption along with 2048-bit RSA key. The most important thing this certificate will help to secure website at the lowest price. Here is the article ( https://www.cheapsslcouponcode.com/blog/which-are-the-best-comodo-positive-ssl-certificate-providers
) which gives information about the advantages & best providers of this certificate
Other than the above certificate, if you want to secure your main domain and unlimited sub-domain with only one certificate hosted on different server then you can go for Comodo PositiveSSL Wildcard Certificate. The main advantage of this certificate is, you can save money and time both as the entire task is managed with the single Comodo Wildcard SSL Certificate.