Your premise confuses me. You've heard the city is plagued with crime and filth, and want suggestins for hotels? Why? You need to overnight there before or after a cruise?

Not sure of your definition of "luxury." Or your budget. You'll find lots of hotels, but not spacious resorts. I personally like Ipanema a bit more than Copacbana, since it's usually a bit quieter (if you're a male in your 20s, I might switch that). There are lots of nice hotels in Ipanema. Check Tripadvisor.comand other sites. I'd probably recommend places right on the beach road. I like the Praia Ipanema, but there are also others. I think the Sheraton is a bit isolated form the rest of Ipanema but if you're just going there for a place to sleep, it'd be fine.