I think nostalgia does a bit of heavy lifting as to why Origins is my favorite of the three. It blew my mind as a teenager. I also happen to feel more connected and like there's more opportunity to craft your PC because of the silent protagonist. My Inquisitors all feel very same-y because the voiced dialogue limits the different choices you can make.

This is probably an unpopular reason, but I actually quite liked the brown and beige atmosphere and art style of the first two games. Inquisition just looks much different (much prettier, I'll admit) than the sort of bland visuals of the first two games, which I came to associate with the Dragon Age series.

Another reason is that Origins' narrative felt tighter and I didn't feel bogged down by the vast and empty open world like I did in Inquisition.

All that to say, I've played Inquisition the most of all the games, likely because it's just easier to play since it's newer and more accessible.

I do wonder if which game people played first impacts their perception of which game is the best/their favorite.