Least Reactive Cookware?
Paulj has a good point. It depends the kind of reactions you are thinking about. Some people only consider chemical stability, others consider thermal stability, and others consider physical stability. You can consider all of them as reactions, or you may only consider one or maybe even none.
For example, Teflon nonstick surface is quiet stable against many chemical reaction (acid and base), but it is not very stable against high temperature. So if you use one at high temperatures, it will degrade (react) quiet fast. On the other hand, a seasoned carbon steel pan is not stable under a bath of acidic liquid, but is very stable against physical abuse and against high temperature abuse. Enameled copper cookware seem to be a odd "marriage" because the enameled surface is really a layer of insulation material, while copper is considered a good thermal conductor. In term of reactivity, it boils down to the enameled surface, and it is fairly nonreactive to chemical reactions, but it is not the most durable against physical absuse or temperature abuse (due to the enameled layer on top of metals).
Least reactive? Well, *none* are very reactive to food, with the partial exceptions of bare cast iron (acidic foods) and aluminum (sulphurous ingredients like eggyolk and onion). Unless you have a severe sensitivity to those metals, tin, and/or the alloying metals in SS, IMO there is no reason for health concern. That leaves ranking the reactive differences between metals as pretty much a theoretical exercise.
I'm less sanguine about the longterm safety of PTFE/Teflon and the ceramic coatings, but not because of reactivity.
If you do not already know, the "enamel copper" lines I think you are talking about are basically clad (usually carbon steel + copper + carbon steel) with enamel sprayed and then fired onto the inside and outside. The reviews are quite mixed as to these coatings' propensity to stick or release foods.
Still, $500 for a set of clad, if it is good, is IMO worth the price. I caution you, however, that some of these enameled copper lines (notably Chantal Copper Fusion) will not disclose how much copper is actually hidden inside their pans, so you must take others' word for how good it is or isn't, or risk buying a pig in a poke. There is one knowledgeable poster here who both sells and uses these and gives them high marks. If they are as good as she says, $500 would not be a waste of money.
Hope this helps,