Let's take the 16GB version as the comparison benchmark version.
We can also compare the tablets based on various factors:
1. Price:
Kindle Fire HD takes the first position with just $199 followed by Nook for $229 and finally Nexus 7 for $249
So Kindle fire HD takes the first position interms of price
2. Processor/Hardware:
Processor is the most important factor for android devices since the android devices are more feature oriented. More no. of processes required to run more no. of apps which in turn require a good RAM to route the processes to processor and prominently a good processor to handle them.
All the 3 devices carries 1GB RAM with Nook having the best memory mangement being the latest one.
The Nook HD has OMAP 4470 processor which is the successor of OMAP 4460 which is being used in Kindle Fire HD. Both are dual core processors except that the processor in Nook HD clocks @1.3GHz which is 100Mhz more than Kindle Fire HD.
Apart from these competitions, Nexus 7 has the Nvidia Tegra 3 processor with awesome Quad cores.
With the best processor the nexus 7 holds the first position without any debates.