Hard question!! Can someone suggest a good CPU cooler (for overclocking) for my micro ATX motherboard?

Hyper 212 has one of the thinnest profile (25 mm thin) or about 50mm with fan mounted. It has quite comparable dimensions as some of the Xigmatek coolers. I believe it won't hit the RAM modules. You can't find anything smaller that can handle overclocking of a 125W processor. Wattage may go as high as 150W when overclocked and running at full load.