I'd recommend trying a few, but regardless of the amount of cores you have, and how much they are overclocked, compressing that amount of video will be slow. As they say on the front page of the handbrake.
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There's an old proverb in the video encoding world: "Speed, size, quality: pick two." It means that you always have to make a trade-off between the time it takes to encode a video, the amount of compression used, and the picture quality
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I would recommend trying Handbrake. It's fairly quick (but may still take a couple of hours for 20 gig of video), but it does give good quality. The downside is it only outputs MP4.
Also, it is slow, but gives good compression and good quality, but you could try the trial version of Premiere. It's over £500 for purchase though. The nice thing is, it comes with Adobe Media Encoder, which has a huge range of output formats. Might be a little excessive though..