cleaning baking sheets

I like to use Bar Keeper's Friend to scrub my baking sheets (actually, I use it on all my stainless steel cookware). Works well though it still requires a bit of elbow grease, especially with baked on stains and you probably won't get it pristine and stain-free as the day you bought, but it comes pretty close.

It's hard to beat a stainless steel scrubber as a decent cleaning tool.

Are you using nonstick coated pans and the nonstick coating is coming off? Sometimes that results in a rust spot where the metal becomes exposed. Maybe you can try under-cleaning it, like you would a cast iron pan. My favorite baking sheet Ive had for 15 yrs and has developed a black patina from all the baked in oils. I for one would have a heart attack if someone put a degreaser on it!:)

You could line them with parchment, silpats or foil when you use them- they're likely still to discolor over time, but much less. Sounds drastic, but have you tried oven cleaner on them? My go to for cooked on food is boiling with a strong baking soda solution, but pretty hard to arrange with a baking sheet.