DAO is the worst by far. It's just brown, variation of brown, and another variation of brown. The weapons and armor weren't all that amazing either, as they were pretty bland. Except the swords Vigilance and Starfang, which are the best-looking weapons in DA imo.
DA2 had horrible weapon designs (those 'greatswords' are freaking tables, not greatswords! the staves were the only decent ones), but I like it much more than DAO. It obviously suffers from repeated locations.
DA:I definitely looks the best graphics-wise, and its art style wins because it's more colorful (so much better than 'wow look how grimdark we are' Origins style) and the environments all look different. Also, the tarot cards are awesome.
Although Val Royeaux did make me want to gouge out my eyes because of the candy colored EVERYTHING.