So far I have yet to find a pair of leg armour that stays cool and fits comfortably over a long period of time.
I started out using the Race Face FR leg armour and liked it a lot. Unfortunately within a year, almost all velcro straps snapped so I had to replace them.
Then I used the Lizard Skins shins and at first, they were really awesome (as in: I couldn't imagine a more comfortable shinguard). The drawback to those is that they're quite bulky, they're really hot and after a few months of riding they wouldn't really stay in place that well.
This is when I purchased the Troy Lee Lopes guards (pictured above). If you're looking for a full shin protection that you should definitely consider something else as they only go halfway down your shin. They were really comofortable, easy to put on and vented but the quality was to be desired (the stitching came loose withing my first ride) and most importantly, they never stayed in place when I crashed on them - so what's the point?
Right now I'm using the 661 Kyle Strait pads - not to follow some stupid trend but because they fit extremely well, are super comofortable and I usually never have any issues with my pedals hitting my shins, so I can get away with using kneeguards only.