ODST is the fourth game in the franchise; most people would argue that to be the worst. Canonical series wise, Halo 4 is yet to be seen. It's just a trend however, it doesn't hold true for all series. Most of the longest running series that are still successful now have a successful fourth title.
Loved the series' first three seasons or so.. but the more esoteric/spiritual it got, the duller it became. That prequel series centering around the first cylon was painful as well.