Cologne is flat and compact on the left bank, so almost every base here is a 20 to 30 minute walk from the cathedral — the real choice is noise, price, and which side of the Rhine you wake up on.
Altstadt & Dom — the postcard core
Everything between the cathedral, the Rhine promenade and Heumarkt: brewery taprooms, the big museums and the Hauptbahnhof all within a few minutes on foot. The trade-off is stag groups on the Rhine-facing streets and near-total sellout during Carnival and the big trade fairs.
Belgisches Viertel — cafés and small shops
The grid west of the Ring around Brüsseler Platz, full of independent shops, coffee bars and small restaurants, with the Wohngemeinschaft hostel in the middle of it. About 20 minutes on foot from the cathedral or a few stops on the Ring trams; calmer than the Altstadt, though the square itself is loud on summer evenings.
Ehrenfeld — former industrial, the cheaper option
West of the Ring, on the S-Bahn at Köln-Ehrenfeld and tram lines 3 and 4, with live-music venues, Turkish and Middle Eastern food along Venloer Straße and noticeably lower room rates than the centre. Ten to fifteen minutes into town by train, so it suits you if you don't need to walk home.
Deutz — right bank, cathedral view, fair-week caution
Across the Hohenzollern bridge, with the best view of the Dom, the Lanxess Arena, the Rheinpark and its own ICE station at Köln Messe/Deutz. It sits against the Koelnmesse exhibition halls, so check the trade-fair calendar before you book — rates here spike harder than anywhere else in the city.