Dittos to Valkyrie Rider
7018 is a low hydrogen rod, there best keep in a super dry storage,,,, or there is a trick to stick the rod for a few seconds on a chunk of scrap to dry it out,,,,,7018's are a real good choice for an all-round structural rod.
6011's are a deep penetrating - all position rod that dose not need super-clean surfaces to perform, they will burn past most anything to get to what needs holding. 6011's dont do thin stuff well.
6013's are a good rod for new mild-steel, and are a good choice for a lot of uses as there easy to weld and make a champ out of most anyone. 6013's are better on the thin stuff, but excellent for multiple pass welds also.
You might want to stock a few (there expensive) NiCi or Nickel Alloy rods if you have any cast-iron work. They require a preheat and a slow cool down, shop wood stoves can do that for you.
Another idea may be a few hard surfacing rods to protect ware areas, never a pretty weld, but there a lot of fun as they will really extend heavy ware equipment. Ask you dealer about them.