Which Captcha is Most Effective???

Hi,

I am rebuilding my website RS Seo Solution because there were some script injection issue in past.

I am using captcha with our all forms (Contact us, request for quote etc.) but still receiving about 10+ SPAM queries per day. Are they submitting queries manually by pass our captcha ???? or My Captcha is not Working and they are submitting garbage queries from any tools ???

Please suggest me that how can i stop this types of SPAM queries ??? WordPress ...