Guitar strings come in myriad variations of wrapping shapes and materials. But I'm always surprised to find the top ones (without windings) are just "plain steel"? Are there any other options there?
As you have observed, most of the string brands differentiate their offerings by varying the way they make the wound strings. But regardless of how creative and different they get with the wound strings, the unwound strings are all the same. Plain steel.
The wound strings for electric guitars can be wound with pure nickel (for a warmer tone and softer feel) or more commonly a nickel alloy (usually 8% nickel 92% steel). Acoustic strings can be wound with "phosphor bronze" or "80/20 Bronze" (these are alloys made from copper and zinc). Windings can be round, flat, or half round and can be wrapped around a round or hex steel core. You can even find "hybrid" strings for acoustic electric guitars with magnetic pickups that alternate windings between a nickel plated winding and "bronze" windings (one string maker calls these "zebra" strings).
But the plain unwound treble strings are always made of steel. The reason for this is that steel has the high tensile strength that can withstand the tension required for tuning without breaking. Thus even the core of the wound strings is steel. The windings do absolutely nothing to add strength to the strings so they can be made of virtually anything. The windings affect the tone and feel and plain steel windings would be very hard on fingers and frets.
The only differences offered in the plain steel strings is that some manufactures use a stainless steel formula to make them last longer or apply a plating or coating to inhibit corrosion. None of this really affects the tone but the stainless steel formulas tend to be harder and stiffer (I don't like them personally).
Elixir claims to use an "anti-rust" plating on their plain steel strings and Cleartone claims their steel strings are protected by "Enhanced Molecular Protection (EMP)".
From a review of Cleartone Strings on LonePhantom.com it says:
Enhanced Molecular Protection (EMP) treatment is typically used on hard disk platters, which require an extremely thin and durable protective coating. The treatment measures less than one micron. The EMP coating actually bonds to the metal, rather than coat in the traditional sense. This also means that it doesn’t flake off like many coatings. Most manufacturers only coat the wound strings, whereas Cleartone coat all strings to ensure maximum string life.
In my opinion all of these claims are more about marketing a theoretical point of difference to get you to choose one brand over another. I have never found any major advantage in any of the specially treated plain steel strings. But I don't leave a set on long enough to rust anyway. I am guessing anyone who cares about good tone would be the same way about not leaving strings on their guitar until they all become dull and lifeless and begin to corrode.
For my purposes, the plain "plain steel" strings work just fine!
For electric guitar, there are also nickel strings that have a warmer tone. Pure nickel will have the warmest tone, but even the 8% nickel wound string will be less bright than the steel strings.
There are also a variety of ways for strings to be wound/wrapped. The way the wrap string is wound around the core string also affects tone:
The profile of the wrap wire also effects tone and feel with: Round wound being the brightest and roughest. Half round being warmer and smoother. Flat wound being the most mellow and smoothest.
On the other hand, acoustic guitars use either brass wound strings (often incorrectly labeled as "80/20 bronze"), which are composed of %80 copper, %20 zinc, or phosphor bronze wound strings which are %92 copper, %8 tin which give a warmer sound.
I have often seen rubber strings on a ukulele, particularly bass ukuleles, and even seen a bass that uses rubber strings.
You can go to this website here for more information on string types: http://www.stringsandbeyond.com/what-strings.html