Frankfurt Airport: Use the S-Bahn, not a taxi: S8 or S9 from Frankfurt(M) Flughafen Regionalbahnhof at Terminal 1 to Frankfurt Hbf takes about 20 minutes; continue to Hauptwache or Konstablerwache for the centre. A single airport-zone ticket is about €6.60. From Terminal 2, allow another 15 minutes for the free SkyLine or shuttle to Terminal 1. Taxis are sensible only with heavy luggage or a late-night group: roughly €30–40 to central Frankfurt, traffic permitting.
Frankfurt-Hahn Airport: Despite the name, Hahn is in the Hunsrück, not Frankfurt. Take the scheduled coach to Frankfurt Hbf; allow roughly two hours door to door and about €21 one way. It is usually a poor choice for a short Frankfurt break unless the airfare saving is substantial.
Frankfurt (Main) Hauptbahnhof: Frankfurt’s main rail arrival point and the practical hub for most hotels. For Altstadt, take U4/U5 to Dom/Römer or tram 11/12; for the centre, use the S-Bahn tunnel to Hauptwache/Konstablerwache. The old town is also a 20–25-minute walk. Do not judge Frankfurt by the immediate station frontage.
Frankfurt (Main) Süd / Südbahnhof: Useful for some long-distance and regional arrivals, and especially convenient for Sachsenhausen and the Museumsufer. S-Bahn reaches the centre in a few minutes; U1/U2/U3/U8 run north from here. Check your rail ticket carefully: not every long-distance train that serves the airport also calls at Hauptbahnhof.
Frankfurt Airport Long-Distance Station (Flughafen Fernbahnhof): ICE/IC arrivals use this station rather than the airport’s regional station. For central Frankfurt, follow signs through the covered link to Terminal 1, then take S8 or S9 from the Regionalbahnhof; allow about 30–40 minutes in total including the station walk. Do not use a local RMV ticket on ICE/IC services.