Sharpie Flipchart markers. Specifically made to be used with chart paper in the classroom. Will never bleed through to the page underneath. Just got some for my classroom and I'm having so much fun making pretty anchor charts!
Pilot Super Color Marker Pen Jumbo Wide Broad SC6600 Lovely big fat jumbo marker pens which are refillable. You can read flipcharts and whiteboards from the back of the room! Brilliant. From Cult Pens.