Districts in Budapest

We're going to start looking at flats for rent. I'm not sure what the differences are between the various districts. Are there any we should definitely stay away from???

Most obvious difference is their distance from your workplace or other places you frequent and availability of public transport that takes you there quickly. There isn't one district that should be avoided entirely. District 8 used to have a bad reputation, but over the last few years many of its dodgy parts have gentrified. Check the street you're considering on google street view, if the buildings are old and in bad shape, then you should avoid it. Generally avoid living close to gipsies, and prefer new buildings built in the last 5-10 years if you want comfort.