What are the best X to use to make Y?

I often wonder, what is the best type of something to use when trying to make whatever it is I'm making.  Did that make any sense, haha!

Well what I mean is, what is the best:

* APPLE to use to make applesauce?  

* POTATO to use to make mashed potatoes (one of Justin's favorites and I don't make very often)

*FISH to use when making fish tacos ... one of my favorites!  
I have an outstanding recipe that uses Mahi Mahi but because I live in such a metropolis I can't find it very easily so I tried tilapia once and it just wasn't the same.  Now the funny thing is the lady at the "meat counter" told me that tilapia was what you wanted to use... it was good but it just wasn't what I was looking for!

What else takes a specific type of something to make a dish really good??  Any recommendations?

I am going to test some different apples, potatoes and fish over the next week so I will let you know my opinion.  I would LOVE yours!

Happy Fall!!

j~