From my own personal experience with a TNT2 (all the way from Detonator 3's till the 10's came out), 5.45 was the fastest (P3, BX motherboard platform). They fixed a few of the rendering issues with some of the version 3.xx drivers and were quite fast. I think the all out fastest may still be one of the late 3.xx drivers but its a couple years too late now to start benchmarking.
From version 6.xx the speeds on the TNT's got hit badly, and the 7.xx drivers sometimes had a few bugs in although some of the later 7.xx versions worked quite well and were almost as fast (but still not as fast as could be).
Keep in mind though this is for Windows 98SE. As for windows 2000 or XP, cant really say. I suppose as the drivers generally have improved a fair bit in overall quality since most people stopped using a TNT card in these OS's, the speed may well have gone up with the latest drivers, but its hard to say without a card and time to test it.