Only you can really say which so I would recommend that you try them all but my guess would be either XFCE or LXDE.
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Xfce or LXDE would be good on that machine. Task-Xfce is not the latest Xfce, you would have to install latest Pheonix or Phinx. Melodie's "OpenBox Bonsai" or Linuxera's E17 "Summer Fairy" are also excellent.