re: What is the best WEED AND GRASS killer I can buy or mix?

Glyphosate is the active ingredient in Roundup. It is the same in many of the off brands as well. I usually buy the off brand, but look for the active ingredient label and match up the percentage to the Roundup you were looking to use.

I usually buy the concentrate and mix double the recommended dosage. If one is good, two is better, right? That will usually kill it down to bare dirt. Gas/diesel is cheaper, but your choice in the type/strength of herbicide you use should also consider what you will do with the land after (growing crops or pouring cement for example).