I liked 4th Survivor, in RE2, and UC was cool too, except they were lazy with the UC version, as they just reused HUNK from RE4, even used the TMP, instead of the MP5.
I think RE4 had plenty of great puzzles. RE5 maybe had 1 or two ACTUAL puzzles, plus a little bit of investigating. RE4 had some great stuff though.
As for Las Plagas, I loved it. It's just another parasite, the other being the Nemesis Parasite, and I like the ganados and even majini. Uroboros...was kind of lame. Nothing special really, it had it's moments, but basically, we've seen wormy/leechy stuff in RE, many times, and it didn't seem to choose people to live in the "new world", it just mutated them, not sure if there was really a point to it.
I think there's more to it than just, zombies vs Las Plagas, but anyway, zombies were great, they are classic, though they could get a little dull, just in the way really, yet Crimson Heads were brilliant, a lot like the infected in 28 Days Later. Anyway, what did you think of all the mentioned stuff?