Opinions on 6013 welding rods?

6013 was designed for general purpose welding on new mild steel.
it is an easy rod to learn to weld with, as it is very forgiving.
6013 can be welded in any position
it is a low penetration rod, and doesn't have the ability to burn off contaminants from the base metal.
so make sure whatever you are welding has been cleaned up really good.

E7018 is the best all around rod, in my opinion
if you buy 7018 in small batches, you'll be just fine.
i doubt that you will be doing any certified work soon,
so the restrictions of having a rod oven and keeping the rod at temperature is not an issue for you.
so long as you are not doing certified work, any 7018 can be placed in a toaster oven @300-350°F for an hour or so to refresh it before use.
restarting E7018 requires a hard thump of the electrode against the work to re-strike the arc

I'm also a huge fan of E6010/E6011, they are my goto rods
i was instructed in comunity college for 2 years by a retired pipeline welder, well laid all our root passes with E6010
both 6010 and 6011 are for when stuff is grimy, rusty, poor fit up.
some inverter welders don't like the 6010, but they will burn 6011

as RJ mentioned E7014 is a very nice rod to burn, very clean looking welds- no need for a rod oven either.
i like to run E7014 on horizontal and flat position work where you need a lot of weld fast