Godzilla (1954) featured a 20 minute destruction scene with Godzilla. This doesn't count the two appearances he had prior to that scene, nor does it count his final appearance as he died. The shorter 1956 edit of this movie for American audiences removes none of these scenes. Though, it might be true that we've never seen that much Godzilla in a single movie since then.
Also, for me, the most distinct and creative looking Godzilla monster was Hedorah, which probably means I'll be banished from the rest of the Godzilla fandom. Mechagodzilla would have counted if he wasn't a robot instead, and the "creativity" in King Ghidorah's design was merely gluing the same modeled head on three different identical-looking necks, and copying the tail and wing design over twice. Awesome? Yes! Distinct or creative? Not until Mecha King Ghidorah changed it up. (Cyborg attachments tend to make any monster design better in these movies.)
Seeing as how the Pacific Rim monsters do have variety in weapons and attack modes, and even silhouette recognizability making them distinguishable from one another, I can't say they have the personality of wet cardboard. Not visually, at least. But they do all kinda sound the same, and that's probably what makes them seem too similar. When I think of distinct Godzilla monsters, I think of different roars or screeches, not really different designs and appearances. Adding as much variety to Pacific Rim's creature sounds as they did their appearances and fight modes probably would have gone a long way in giving them a more distinct "personality."