When I started stitching needlepoint we had very few threads - three different wools, cotton floss, cotton perle, and one metallic. Thirty+ years later we have so many threads to choose from that it's sometimes overwhelming!
I tend to select threads I am most comfortable with for my own stitching, because when I am not working I want to relax. When creating a stitch guide I am more likely to select what works best for the stitch and canvas, even if it's something I don't personally like. I always try to find ways to use new threads if possible. One great thing about doing a stitch guide is that I can try new threads and add them to my "favorite" list.
I am starting with Rainbow Gallery because they have such a variety. Rainbow Gallery has a lot of information about their threads, including how to use them, on their website. Many of these threads are intended for use with decorative stitches, particularly long stitches, to get the full effect.
I encourage you to experiment - it's a relatively cheap thrill to discover a "new" thread, or a different way to use an old favorite.
Alpaca - great for animals. I have all of the colors in my stash.
Angora - stitches up nicely. Consider for animals with long hair, caterpillars (Wild Lime color), baby birds, sweaters, scarves, and Santa's beard and hair.
Capri - a new thread with vibrant colors. Very useful when you want a matte effect to offset a shinier thread.
Crystal Braid - this is particularly good for ice, snow, suns, and moons.
Entice - I am loving this new thread and have been using it a lot. The colors are outstanding. Use short lengths and you won't have any trouble with it.
Flair - excellent for glass (windows, glasses), water, ice, and very useful for holiday ornaments. The greens make nice leaves and stems. And you can "destroy" this and couch it onto the canvas to make foam, smoke, and clouds. There are a variety of oranges that I like for snowman carrot noses.
Frosty Rays (Petite) - when I want a metallic look but don't want it to be real shiny. I have used this for leaves and flower petals.
Fyre Werks and Fyre Werks Soft Sheen. When you want a metallic that's eye-popping, Fyre Werks is perfect. I like the soft sheen version for a more muted metallic look. Both are outstanding for Christmas ornaments and packages.
Fuzzy Stuff - I tend to use this for things like spiders and the brims of Santa hats. I have even used it for Santa's beard. Some of the colors are really wild and you can create all sorts of interesting effects with them. I often use a simple stitch like a Byzantine or Satin with this thread, especially the black, because I am going for texture.
Glow in the Dark threads - Rainbow Gallery has several of these in different threads. As a dedicated Halloween stitcher I was really happy when I found this in one of my most-used threads, Silk Lame' Braid.
Grandeur - my favorite #5 silk perle. I would like to see more shops and designers support this thread so we can keep it around. I often substitute this for Perle Cotton when stitching from a chart or stitch guide.
Neon Rays and Neon Rays+ - this is one of those threads I like to use even though it has to be laid and it takes time to make it look right. Putting them next to matte threads really brings out the shine. I think this looks best with longer stitches (at least over 2).
Rainbow Cashmere - there is no substitute for this thread when you are stitching a "cashmere" sweater or scarf on a canvas. It feels good too.
Rainbow Linen - I really like this thread. It has a texture and slightly matte finish that work well next to shinier threads. Great for brooms, textured clothing, and even witch's hair.
Silk Lame' Braid - the best new thread to come along in a while. It stitches smoothly and there are so many colors. This is another thread to use when you want some metallic but don't want it to be too shiny. The petite version makes a great background thread on 18 count canvas, particularly with an open stitch.
Splendor - a great workable silk with a lot of colors. I like that you can usually find a color family with this thread and several others in the Rainbow Gallery line - very useful when stitching in one color with different textures. And you can "fuzz" this thread after stitching by gently brushing it - an interesting choice for birds' wings or baby animals.
Very Velvet (Petite) - oranges and blacks for Halloween designs, greens for leaves, excellent for "suede" shoes and belts.
Wisper - another great thread for animals and birds, anything that needs to be fuzzy with a matte finish, and Santa's beard and suit.