Bekele's superior track speed and his string of short-long victories give him the overall edge in my mind, but I much prefer to watch Ngugi, who was an all-terrain beast. Looking at him run over and into the snow of Boston, I wonder how many times in his entire life he had even felt snow on his face or under his feet before that race. And at the other end of his range, his Olympic victory in the 5,000 was the brilliant execution of a race strategy by a runner who understood his limitations as well as his strengths. It would have been interesting to see Ngugi at his best in the heat, humidity, and sloppy mix of dirt and sand in last year's race in Mombassa. He actually was a bit like Tadesse, with somewhat limited finishing speed but the strength of a bull.
I also wouldn't count out Tergat as a candidate for the GOAT of cross-country. His dominance in that domain was perhaps discounted too much by his losses to Geb on the track.