Ignoring the "bird" solution (while birds are the closest descendants of dinosaurs, it stretches the English language excessively to say that they really are dinosaurs in the same way that it stretches the English language to say that humans are chimpanzees), the dinosaurs most likely to manage if revived would be aquatic dinosaurs. Technically, they are "marine reptile megafauna" rather than true dinosaurs, but they fit the general sense of the word dinosaur better than birds do.
Pliosauroids (illustration above per Wikipedia), which had large heads with very strong jaws and short necks, inhabited an ecological niche similar to large sharks, would be one strong candidate.
Nothosaurs (illustration above via Wikipedia), which inhabited a ecological niche similar to seals, would be another.
The blue sea environment is probably the most similar today to what it was in the era of the dinosaurs of any ecosystem on Earth. Both the flora and fauna of terrestrial Earth are mostly dramatically different from what they were in the dinosaur era, but the blue seas and oceans of today are far more similar to what they were like then, and will become even more similar with global warming.
There are lots of marine species that are still undiscovered and there are other "living fossils" like the coelacanth that were thought to be extinct only to be recently rediscovered, so these species might be discovered long after they were reintroduced.