I also favor neutral films. My favorite was Kodachrome 25, but that was only 35mm, and alas it is no more. On the other hand Astia in medium and large format is a beautiful neutral film--a little on the cool side, so I usually use it with a slight warming filter like an 81A or KR1.5 for a neutral result. The new Astia 100F looks even better from the examples I've seen, but I haven't used it myself.

EPN is probably the most accurate film out there, particularly with strobes or short exposures outdoors.

Fuji MS 100/1000 is a fairly neutral film at EI 100 and 200. I've shot a good deal of it in medium format, and even though it's no longer made, Freestyle may still have some in 35mm and MF cold-stored for cheap. It's grainier and less saturated than Provia 100F or 400F, but arguably has a little better acutance.

I haven't used the current RSX, but I used to use Agfachrome 100, and this was a great, relatively neutral film that favored pastels. It could usually print straight onto Cibachrome without any masking. I think most people find Ilfochrome difficult because they are shooting oversaturated, excessively contrasty slide films to begin with.

I'm not a fan of Velvia or the saturated/warm films from Kodak, though I've seen some nice images with Velvia in very flat light or in commercial uses where a strong "graphic" look with bold primary colors is required to fit into a magazine layout.

In flat light, my preference is Provia 100F pushed one stop. I've shot a lot of Kodachrome 64 in 35mm, but I've been phasing it out as processing options become more limited. I guess I'll replace it with Provia 100F, since I already use that. I also shoot Provia 400F when I need more speed.