Tergat is definitely a great runner - but he certainly didn't win 6 straight XC titles, he won 5 between '95 and '99. Mourhit was a caught cheater in '02, but he still won in '00 and '01. In '00 Tergat was 3rd anyway, so it's not like he can claim the gold that year, even discounting Mourhit.
As one who has never won Olympic Gold, he certainly can't be classified as the best - we've been down this road before!! - and Bekele is the best (at this moment) on the turf as he matched Tergat with 5 and won the short course for 5 as well - something that Tergat didn't even once accomplish, not that he tried to either.
I'm not sure why Geb wasn't as succesful on the turf, in the early years it was lack of depth in the Ethiopian squad versus the Kenyan team tactics, after that I think he just had a distaste for it, and competed indoors for a few years instead. Perhaps his achilles troubles(often a chronic problem for him) convinced him not to bother - it was Tergat's domain.
In this day and age of specialization it's tough to be great on the track, turf and roads. Neither Tergat or Geb have won a significant championship on the roads (Marathon anyway) as yet, but you've got to give the Track to Geb and the turf to Tergat - but Bekele could outstrip both if/when he moves to the Marathon, which I believe he will do when Geb retires. As of now the edge might go with Tergat because his WR in the Marathon, but that is just a borrowed thing (as was either of their 10000m WR)- Championships are forever.