I'd go with either the TNT2 for a more period-correct, "slumming it" feel, or the 9800 Pro for high FPS, AA+AF bliss.
I spent some time with a TNT2 Ultra on P3 1000 Windows 98 system a few years ago and enjoyed the heck out of it. It wasn't the fastest, but the VGA signal was incredibly clear and strong (Diamond Viper V770U), the 16-bit dithering was excellent, and all the games just looked and felt right. Nothing weird or out of place. This was with ForceWare 30.82.
ATI's R300 GPUs are also a real treat in Windows 98. Anything above the Radeon 9550 is ridiculous overkill, none more so than the 9800 Pro. But that excess power can often be put to good use by enabling antialiasing and anisotropic filtering. With the exception of fast GeForce 6 series cards (not the 6200), no other Windows 98-compatible video cards can deliver comparable combinations of frame rate and image quality. And ATI's latest Windows 98 drivers for R300 are remarkably good. Very few compatibility problems. Not sure how much AGP 2x will hold the 9800 Pro back, but it can hardly fail to be way faster than the TNT2 in any case.
As for the the Ti 4200 and 5200, I've never liked those in Windows 98 systems. Maybe it's just because Need for Speed: Porsche Unleashed has font rendering issues, but GeForce 4 and FX series just never feel like a good fit for Windows 98. Not terrible, but it just seems like there are better options.
For maximum fun, though, why not try out all four and compare? 😃