Personally, I don't think you need to highlight plans by their speed as the table is rather easy to follow. But I do have the following comments:
- Do you really have plans with speeds ranging from KB/s to GB/s? If not, I would really try to settle on a consistent unit of measurement across all of them.
- If possible, I would make the table sortable by the user. That is, using your sample, allowing the user to click on the header to sort your plans by download or upload speeds, or by price, etc. Or perhaps just provide a drop-down menu prompting the user to Sort plans by and have it default to "Best download speed".
Below is a mockup I did as an example:
The above provides the best user experience because it empowers them to differentiate your plans based on what is important to them. Note that the above example includes both the sorting options in your headers as well as the "Sort plans by" menu, but you wouldn't need to implement both.
In terms of the actual question though, you could use some sort of icon to indicate the speed. Below is a quick mockup I did of how this could work:
Of course you'd have to use appropriate icons. But, as I said earlier, the best approach would be to do something like the first mockup above.