In this paper, we present a novel ultra-compact Physical Unclonable Function (PUF) architecture and its FPGA implementation. The proposed Delay Difference PUF (DD-PUF) is the most dense FPGA-compatible PUF ever reported in the literature, allowing the implementation of two PUF bits in a single slice and provides very good values for all the most important figures of merit. The architecture of the proposed PUF exploits the delay difference between two nominally identical signal paths and the metastability features of D-Latches with an asynchronous reset input. The DD-PUF has been implemented on both Xilinx Spartan-6 and Artix-7 devices and the resulting design flows which allow to accurately balance the nominal delay of the different signal paths is outlined. The circuits have been extensively tested under temperature and supply voltage variations and the results of our evaluations on both FPGA families have shown that the proposed architecture and implementation are able to fit in just 32 Configurable Logic Blocks (CLBs) without sacrificing steadiness, uniqueness and uniformity, thus outperforming most of the previously published FPGA-compatible PUFs.