The short answer is that there is no single best Linux distro for a netbook. But not all hope is lost, you can find an answer for your specific use case.
If you bought or intend to buy a netbook with Linux pre-installed chances are that the installed operating system supports the hardware well or is even optimized for it.
Operating systems like Xandros and HP Mobile Internet were designed for this type of low performance device with a small screen. Unsurprisingly, people from the Linux community created new distros too, some targeted at specific models and more general ones, you find many of them in the list of Linux distros covered on this site.
Since netbooks passed their peak, many netbook specific distros are not further developed, but due to the abundance of small screen mobile devices there are active Linux projects well suited for netbooks.
There are a few key aspects to what makes an os suitable for netbooks, independent of personal preference:
Apart from those, personal preference and usage scenarios are key to finding what is the best os for you. I've written about things to consider in this guide to choose the right distro.
I don't want to pick a certain distro here, because may preferences are probably different from yours, but if you have a question about a certain model, reach out and I'll try to give a more specific recommendation.
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