Age of Empires one and two.Especially the second one.I've been playing these games for 10 years,yet they never get old.I still play regularly to this day.
Are you looking for more realistic games? Most fun? Easiest to play (more relaxed)? Or a combination.
The Total War series, especially Medieval 2: Total War, are the most realistic RTS I have played. The characters look great, the battles are awesome, and the campaign is hugely fun. The problem is that they get old and boring after about 3 months.
For most relaxed and easiest to play I would say Age of Empires. Take your pick, they're all great. The game play is simple, the units look cool, and if you get tired of Scenarios you can create your own map. The problem with Age of Empires is that sometimes it feels too simple. Lets face it, those battles don't look that cool. Overall it's a great game.
The most fun RTS I have played is Rise of Nations. Even though you only have three nations to work with, the game play and battles (although not very realistic) are fun. Rise of Nations is a lot like Age of Empires, pretty much the only difference is the units and the civilizations. I would say Rise of Nations is better.