Best Recipe for Sourdough Experimentation?

Maybe try a 1 2 3 bread > 1 part by weight of starter. 2 parts by weight water, 3 parts by weight flour.

if you like to experiment, get a kitchen scales that weighs down to one gram.  Then all you need is a basic recipe in % to play with and use baker's percentages.  Baker's% is based on the amount of flour, always 100% and all other ingredients relate to that amount.  ( they will not add up to 100%).  

A basic recipe might be 100% AP wheat flour with 50 to 65% water, 1 to 2% instant yeast, 1 to 2%  salt    

That may translate into. 500g AP, 325g water, 5 to 10g salt, and 7g instant yeast is an average recipe. Less gives you longer fermenting times (more flavour over time) and you can drop it down to just a pinch if you desire.    

With a stronger flour, the hydration may be increased. Whole flours may also absorb more water.  Flours can be combined and combinations explored. A calculator is handy, real or app.  

Another popular recipe to play with is the 1,2,3 sourdough, a ratio recipe based on the amount of sourdough used with 2% salt on total flour weight.  Its based on a 100% hydration starter. Dough is 71% hydration. 1,2,3 are the weights of sourdough, water, and flour.  So you can see, any amount of sourdough starter can be made into a loaf.  It's a fun recipe to tweak.  1+2+3=6. So a desired dough of say one kilo divided by 6 gives the sourdough starter amount.  There are many posts here using this dough formula.  One super long one is a current community bake that is stuffed with ideas and variations.  

Take your basic recipe get to know it, Then add in ingredients, sugar, or fats or eggs or spices or whatever you desire to change flavour or texture and play with different methods of combining, timing, temps, etc. to see what happens. Tip: switching out water for milk usually involves adding about 12% more milk to make up for the milk solids in the milk. Egg whites should be part of the total water. Sugar will not add to the hydration but tend to act like a liquid in the dough.

If you have a favourite size tin or loaf, perhaps starting from there.  Let us know, we'll help you figure out a basic recipe that might be a good place to start.    :).  Welcome!  

The suggestions made already are really good ones.

In my opinion, as long as a recipe is not too complicated, and it makes bread that you like to eat, you can experiment on it. But the simpler it is, the more room for experiment you have.

Start with all-white only if you like to eat all-white.  Otherwise, start with a bread that is your ideal.  Why not?  Just because it's all-white doesn't mean it will teach you more than 10, 20 or 60% whole grain. 

Edit (i.e, further thoughts):  Presumably you are keen to understand better what's going on in your baking.  There's more to that than variations in one formula.  Your hand work, timing and temperature are just as influential on outcomes.  So here's another simple tool that I've learned recently because I've been lucky enough to use it:  Try to bake more often than once/week.  If I can bake on a few consecutive days, I find my hand and eye memory serves better to reinforce understanding of dough behaviors than were I baking less frequently.  Obviously there's some age-related memory fragility involved ;-), but I'm certain it's more than that.

I like your pretty (and happy of course - on a beach) dog.

Tom