I like Pokemon Diamond Version and Pokemon Pearl Version for the Nintendo DS.
The Sinnoh region is well designed and fun to explore, while having a cool and unique atmosphere. The OST is great and extremely memorable, its mix of classical and jazzy music feels very tonally appropriate for the vibe the adventure is going for. The Pokedex is admittedly rather limited, but I like the Pokemon that were included and have gotten a lot of use out of most of them over the years. I enjoyed the return of the day/night and other time based features, they make the world feel immersive and keep me coming back each day. While I do feel that Platinum ultimately improved these games greatly, I would still like the Gen 4 Sinnoh games even without Platinum since Diamond and Pearl still contain 95% of what I like about them. This is all just my opinion, and you're free to disagree.