Hi Cody,
There are some factors involved here... But since you asked a very general question, I'll give a very general answer.
For DC+ power supplies E6010 cuts through contaminants best... Rust, mill scale, paint, oil, junk of any sort.
For AC power supplies E6011 is best.
Now if you have a code or specification to follow, specific contract documents, or a base metal other than plain carbon mild steel, you need to refine your question and the answers might change.
Edit:
It must be added that naturally; removing contaminants prior to welding is always preferred to welding over them... Great electrodes and great skill will not *always* overcome contaminants on base metal.
Just a guess; would this happen to be oilfield production tubing and sucker rod? Which would be 4140 or 4150 chrome molybdenum steel. Then cleaning the weld area would be advised and weld all the way out with low hydrogen (E7015, 7016, 7018, 7048 or 8018 or 8045) electrodes.