What is the best carpet cleaner?

My fiancé and I recently decided we should purchase a carpet cleaner (like the actual machine). About 3 months ago I brought home a chihuahua puppy and she has had a few accidents that we spot clean when we can catch it (side note: I did hear that Chihuahua’s have a very good sense of smell and can smell ammonia in cleaners which they think is pee so they pee more, so an option without ammonia in the cleaner would be wonderful). On top of that, we live in an apartment that is 90% carpet and feel as if the carpet is starting to smell. We are relatively clean people but this is something we haven’t encountered before. Rec’s?