Here I will show the sites I'm using at the moment, I researched and are paying.
This list is decided that the use and may vary from time to time.
You decide what you want to use, and the best for your business prospects and / or earnings.
Rating PTC sites: (rate taken into account the ptccentral.com)

Elite - Recommended -> Online and paying for more than two years.

Legitimate -> Stable and will Paying more than 1 year, paying no major problems.

New -> In testing phases, online will less than 1 year but paying.

Why use several PTC sites?
I am personally using various PTC sites because they do not like to depend only from one site to make money, and gain multiple sites.
PTC sites should also be seen in a way to make money in the medium / long term, because it takes time.
But read the articles of each of the sites, and choose which will want to use.
 
Need to invest money?
No. You do not need to invest money, but I recommend to invest what you can, because this will speed up the process of renting referrals or make membership upgrade.
If you prefer not to invest money, know that takes a long time for it to start renting referrals or earn more, but not impossible!
If you prefer to invest taking particularly care for the new sites because the sites can not pay, so it does not invest more than you can lose!