Inspired by the question Bach's Well-Tempered Clavier Order, I thought I'd ask what order people would recommend learning to play Bach's Inventions and Sinfonie.
I once asked this of my piano teacher. The response I got first suggested obtaining a good edition with all the ornamentation properly explained (and printed above the relevant parts rather than right in it so that you may learn it first without). She personally recommended the Alfred edition edited by Willard Palmer.
Next, you will likely do best to begin with the two part inventions rather than sinfonias, as the sinfonias are more difficult on the whole, although you might find No. 6 easier than several of the two part inventions.
As to the inventions, the best ones to start with are ones with less complex rythyms and fewer ornaments. No. 4 and No. 6 are good ones to start with, but you might be fine starting with others such as No. 1 or No. 8, if you play partially by ear and are assisted by familiarity.