My favourite games are:
1. The 'Close Combat' tactical wargames series. The games are old now (the first one was out in about 1995, at a guess) but the gameplay is addictive. You have a one dimensional, top down, view of a battlefield and have to use your company - composed of mixed units such as armour, engineers, infantry, etc to control victory locations. The units are not tactical icons but little blokes, tanks, etc.There are three main games, based around WW2 - Arnhem, Russia and D-Day. The games can still be found online, via a company called Matrix Games;
2.Stronghold and Stronghold Crusader (the sequal games lost the plot but the first two are v good) I've lost weeks of my life building and defending castles;
3. Silent Hunter 3, with 'Grey Wolves' mod package. You command a U-boat and can play a Captain's career or try real missions like Prien at Scapa Flow;
4. Medieval Total War 2 - I'm on a self-imposed break from this after, two months ago, playing from 8pm until 5am, on a work day, without realising what time it was. I was in campaign mode and thought that the bird noises were sound effects, when they were really outside...