Dry gel highlighters with fluorescent colours and with twister mechanism whereby it is not necessary to resharpen them.
Gel highlighters slide smoothly on the paper and do not wrinkle or smudge it. In addition they do not percolate through the paper. Therefore they are very practical for use on fax paper or thin paper.
And if you forget to put the cap on they do not dry! Because of their translucid wax leads.