[Raptor1]

All of your numbered questions are entirely subjective, you can't know what is boring, repetative or useful until you try it yourself IMO...

If I were to recommend games, here they are:

Air: As Dowly mentioned, for WWII I would get either IL-2 for a survey (Multiplayer also) sim and BoBII for a study sim. Outside WWII, Red Baron 3D for the Great War and MiG Alley for the Korean War are both old but still excellent sims. For a modern simulation, try one of the Falcon 4.0 variants (Allied Force, and OpenFalcon).

Naval: If you want a strategic type game, you have SES' Distant Guns 1.5 and Jutland about the Russo-Japanese War and WWI respectively, which I highly recommend. For a modern game, Harpoon 3 ANW is very good and highly detailed but lacks graphics; Fleet Command is older and less detailed, but easier to get into and is still a pretty good game.

For a single vehicle simulation, DW is definitely a good game for a modern setting, and of course SH for WWII.

Ground: Too large a variety to properly list, so I'll just put in my favourites. For a tactical game, the Close Combat series is definitely excellent if slightly dated. Operational-level wargames (My favourite genre in strategy) include The Operational Art of War III, which can take some getting used to but beats everything else for versatility, with scenarios ranging anywhere from modern to Pre-WWI; there's also the Airborne Assault series (Highway to the Reich, Conquest of the Aegean and the soon to be released Battles from the Bulge), which is also excellent, highly detailed and realistic.