Why you shouldn't do this
Since this is longer than a comment, I will add it as an answer even though it's an answer that recommends not doing what you are asking.
There are both conceptual and practical reasons for not doing so. Of course, if you find yourself producing particular kinds of documents with a similar structure, there may be grounds to create a custom class or package for that kind of document. But if you just find that in most of your documents you load the same sets of packages, I would not recommend this, but instead use your editor to save some sort of standard template documents.
Conceptually, packages should add a specific set of functionality to a document of any kind.
Document classes, on the other hand, should provide the markup for a particular kind of document.
See the following question for some discussion of the distinction.
Now what you are suggesting seems to be neither of these, but rather is just the list of
\usepackage commands (perhaps with some added macros) that you commonly use in most of your documents. This doesn't meet either of these criteria, and so should probably not be implemented as a package or a class.
Now if you do have needs for a particular functionality then of course creating your own package makes sense. For example, I have a package which I load when I write letters which contains macros for my signature etc. I also have created a class for writing conference abstracts, since they require very condensed formatting due to word/page size limits.
Even if you use some packages very frequently, it's likely that you will not need every package you load in your default preamble in every document. So you will effectively be loading packages you don't need. For smaller packages this isn't much of an issue, but for large package like TikZ for example, this will increase the compilation time for those documents.
Creating a default preamble effectively hides the packages that any document is using from you. This means that you will have to remember which packages you always load if you ever need to look for documentation for example or to debug an error that might arise from one of the packages. This may not seem like a big deal, but in my experience it can get annoying quite quickly, because over time you will forget what's in your standard preamble. It makes more sense to have exactly the packages you need loaded explicitly in the preamble of each document you write.
Creating a default preamble also makes your documents much less portable. You can't just put them on a flash drive and use them on someone else's computer, unless you also copy over the preamble package. Nor can you upload them to journals or share them with collaborators (without sharing the preamble code too).
What you should do instead
If what you want to do is just have a bunch of packages loaded and a few other things, you should use your editor to save a template. I use TeXShop on the Mac, and it allows me to create "Stationery" files which are document templates which have a standard preamble which I can define. When I open a file using the Stationery I get a new blank document with anything I like already there. Then I can delete the packages I'm not using easily to create each document so that it conforms to the criteria above.
You can do the same in other editors too. TeXStudio has a "Make Template" function (in the File menu). Create a new document with the preamble you use the most and then choose "Make Template" from the File Menu. A dialogue box will show up which allows you to name it, and add a description (and even a licence). Once that's done, for subsequent documents you can use "New From Template" to open a new untitled document with that basic structure already filled in.
TeXworks also has such a function. You can create template files and store them in
<resources>/templates and they will be available using the "New From Template" menu item.