Glad to hear that I can skip Digimon World DS. I've been looking for it locally for years, but I've never managed to find it.
Of the three Etrian Odyssey games on the DS, III is the best. I and II are good, but they're not as well balanced as III, even though III has some issues with statuses and binds not being as effective as one would hope. The music and story are both better than I and II as well.
As for Izuna, 2 is more forgiving because both party members have to die for you to lose and, while both party members are alive, you have a rechargeable tag attack that hits everything on the screen. Both mechanics give you more options when the game pulls some BS. However, 2 is a continuation of the story from 1, so play 1 first if you care about that. Izuna 1 and 2 (and Etrian Mystery dungeon) a great middle ground between the Pokemon mystery dungeon games, which are very easy for the genre, and the Shiren games, which tend to be some of the most difficult in the genre.
Of the five, I think I'd say Etrian Odyssey III is the best.