It really does depend a lot on the particular instrument - and the way one approaches that particular instrument...
For my Carvin fretted four-string, I'll experiment with a new make & model from time to time - but almost always string up with nickel roundwounds, and always use light gauge - .040-.060-.080-.100. It's a really nice mainstream rock/pop axe.
For the last 2-3 years, I'd been playing GHS Boomer B-52s, with reasonable satisfaction. But as of tonight, I'm trying out a new set of Dean Markley Bass Blasters. So far, so good. Let's see how they age. (For one thing, the price is right: $20.00 + change for TWO SETS @ JustStrings.com)...
I've played with stainless steels before (GHS Super Steels & MTD strings), but can never seem to get used to their "clackiness" of tone. The nickels sound so much better.
My fretless Carvin LB70F (on order) is getting set up with a set of DR Lo Riders (nickel). Though I'm sure that some of the flatwounds (TI, Pyramid Golds) are wonderful for a really warm, smooth fretless sound, I also like a fair bit of upper mids & rich overtones in my sound, so I'm partial to compression wounds, ground wounds and the like - especially for fretless...
MM