My heart's answer, as any reader could easily guess, is "why pick just one?" but on an e-mail reflector to which I subscribe, a member was asking after the best, most basic and most portable typewriter.
If size is an issue and simplicity the key, the three-row Underwood that graces this blog's heading would be my choice. It's built like a tank, is remarkably free of extras and has the smallest footprint of any practical typewriter.
If weight is more an issue than size, one of Remington's more basic portables from the 1930s and '40s would be my desert-island choice.
Past pen or pencil and paper, what would you take? Typewriter through laptop through whatever....