1 is definitely a low-tier DQ for me, but DQ1 still has...novelty in its simplicity. I like it. But I don't love it. DQ2 potentially has all the makings of a classic but is highly flawed. I still enjoy the re-releases SNES and GBC versions a lot more than DQ1 though. DQ3 is really when the series gets into something special, and this continues onward with 4 and especially 5.
I can see why you'd think the series peaked at 5, however. 6 and 7 are basically reiterations of earlier ideas (namely DQ3 and CT). That being said, 7, I think has all of the best attributes of Chrono Trigger more so than DQ6. At its best, it's certainly better written than CT, and I think it explores the sense of time travel better than CT did. The main problem with 7 is its bloat, which CT simply does not experience.
In regards to VIII and IX, I really love both, though I'd place IX as a mid-tier game, and VIII as a low-tier game. Both are easily above 1 and 2, however. Though 2 has more memorable gameplay than 8.
It's pretty clear that they wanted to do something different with 9, and I can't blame them. I'm looking forward to 11 to see what they've got planned, but I'm more worried about Sugiyama than anything else.