Not sure about lumping tactical with strategic wargames but for tactical combat games, it is no contest, the Combat Mission series is the best of the best. As far as I know it is the only computer wargame where you have the option of playing it real time or pbem. Plus the pbem is not server based.
I have been playing this series since the original Combat Mission games came out 20 years ago and they keep getting better and better. You can set it at whatever level of realism you want and I play it at Warrior level which is one down from the most realistic level. It has the best replay of any tactical wargame as you can view the action from any angle and any part of the battlefield you want. Although there are some decent tactical games like Close Combat they are real time only which means you have the issue of finding a time that works for you and your opponent, connecting up, plus with real time you're always going to miss some of the action on the battlefield.
Check out the videos on You Tube of game play for Combat Mission and you will be impressed.
I currently have Combat Mission Battle for Normandy, Combat Mission Fortress Italy, and Combat Mission Final Blitzkrieg. I'm also playing one of the battles in the demo for the just released Combat Mission Shock Force 2 and 21st century weaponary is deadly to say the least.