I've been using Dunlop Delrin 0.96mm for years, last year I could not find any and purchased another style of Dunlop, not quite the same. I did a search today and came across a UK website simply called
www.guitarplectrums.co.uk  to my excitement discovered I can still get the Dunlop Delrins.
I've also ordered a plectrum made from Buffulo Horn in jet black, it looks interesting if anything else. They have also ones made of bone and wood would you believe
Man I did not realise the choices. It really does make a difference in not only how the guitar sounds, but how it feels whilst you are playing.

Plectrum Choices:

    Celluloid. Historically, this was the first plastic ever used to produce picks[citation needed], and it is still of some use today, especially for guitarists aiming for vintage tone.

    Nylon. A popular material, it has a smooth and slick surface, so most manufacturers add a high-friction coating to nylon picks to make them easier to grip. Nylon is flexible and can be produced in very thin sheets. Most thin and extra-thin picks are made out of nylon[citation needed]. However, nylon loses its flexibility after 1–2 months of extensive use, becomes fragile and breaks[citation needed].

    Acetal. Acetal is a highly durable class of plastics. Delrin is DuPont's trademarked name for a type of acetal. Delrin is hard, glossy and durable, and can also be doped to produce a matte texture. The friction between a steel or nickel guitar string, and smooth, glossy acetal is very low. Glossy delrin picks literally glide across the string and therefore have a fast release, producing very little pick noise, while delivering a rounded tone emphasizing the lower order harmonics[citation needed]. This is my preferred type, when you strum an acoustic guitar you don't get the sound from the pick, excellent for micing.

    Ultem. This plastic has the highest stiffness of all plastic picks[citation needed]. Produces a bright tone, popular among mandolin players.

    Lexan. Glossy, glass-like, very hard, but lacking durability[citation needed]. Used for thick and extra-thick picks (> 1 mm). Usually has a high-friction grip coating.

    Acrylic. Tough, light, seamless polymer with great resistance to impact and weathering. Does not yellow, brittle, or crack. Can be molded and cut to almost any shape and thickness

Metal- Picks made out of steel produce a much brighter sound than plastic. They do however wear the strings quickly[citation needed] and can easily damage the finish on the guitar if used for strumming, especially on acoustic guitars. Some metal picks are even made from coins, which give players a unique tone as the alloys used in various coinage from around the world will vary greatly.

Wood- Each guitar pick made of wood has its own unique properties and signature sound as a result of differences in density, hardness and cellular structure. Most wood picks will produce a warmer tone than plastics or metals. In order to withstand the rigors of picking and strumming only the hardest woods are used for picks, including hardwoods like African Blackwood, Bocote, Cocobolo, Lignum vitae, Rosewood, and Zebrawood.

Glass- Glass is relatively hard and heavy in comparison to metal or plastic and therefore produces a greater range of tone[citation needed] than these materials. Glass can be polished to a smooth or rough texture depending on the grit of sandpaper used. Likewise, factors such as size, shape, and weight have a much more dramatic affect on the overall tone making each individual glass pick sound and feel unique.

Other- Agate picks range in thickness from 1mm (very rare) up to 5mm, and are extremely inflexible. As they are harder than the metal guitar strings, they resonate the strings more completely[citation needed][original research?].

    New Tortis is an alternative to natural tortoise[citation needed], made of polymerized animal protein. It is hard, smooth, thick, and has only slight tip flexibility.

   Felt picks are mainly used with the ukulele.

Interesting topic, denis.
After a few experiments with other plecs , such as Ibanez plecs, I found back to the good old Fender 351 medium picks ( hard for bassplaying) which I played decades before and play for years again now.
Back to the roots.
http://www.guitarplectrums.co.uk/fender-classic-celluloid-351-tortoiseshell-medium-guitar-plectrums

I started out on the standard Fender picks, back when I first started playing the choice was either Fender Plastic or Dunlop Nylon. Have you tried any other picks?
It's a bit of a con really the ones you get are not real tortoise shell, my guitar teacher said real tortoise shell was the best back then in early 80's.
One thing I notice is that the thicker the pick is the fatter the sound, I use the fattest I can use comfortably, plus the smother the surface the cleaner the sound.
Edit** Apparently it became illegal to sell real Tortoice picks in 1947 - Shame I'd really like to try one out, but there is an alternative here http://www.tortispicks.co.uk/ They are said to be like real tortoise shell

i use different ones for different things.  Dunlop green tortoise style pick for some stuff. I like thin picks for strumming AC guitars. I've used quarters and dimes for certain sounds.  Fast picking thick sounds I prefer thicker picks. Lots of strumming type songs I like thinner picks. Sweep picking I like in between a thin and medium pick( that varies on speed and intensity).
For Jazzier sounds I like stone picks. http://stone-guitar-picks.com/Jazz-Guitar-Picks.html

Some pro guitars say go for the biggest hardest possible.

Why?

Because you have more tonal control if you learn how to use them.

Yeah, I remember reading an article several years back where the author made a strong case for using hard picks - he claimed that, if you utilize the correct technique you can even use "soft brush" rhythm effectively without resorting to a thin pick. I haven't mastered that, so I tend to go back and forth between several thicknesses (sizes, shapes, etc.) of pick based on the instrument I'm using, or whether I'm playing more lead than rhythm, or it can be based simply on my mood at the time....