Age of Empires the way it should have been. You get to play through not 4 or 5 measly ages, but from the stone age all the way into the fictional future! You can have your riflemen fight off waves of enemy swordsmen, or upgrade your infantry to cyborg soldiers. Walls that have a purpose for the first 10 or so "ages" then becoming (understandably) obsolete when long-range artillery gets invented. No game has every sumarized the evolution of warfare so perfectly as Empire Earth 1. Both EE2 and EE3 ditched RTS for the "flavor of the month" style of RTS that was becoming popular -- "Sector conquest". Lot of good that did ya!
Three was a game that tried to improve on "2"s great gameplay by making it 3D (like every other 2D game of the time) and failed miserably. But the extra dimension alone cannot be blamed, they tried to improve on the original formula but ended up ruining it RA3-expansion style. It was a valiant effort and may be your "cup of tea" but I still remember my extreme dismantlement in buying this for $50 back in the day only to find it wasn't as good as Lords of the Realm 2. (If GOG staff is reading this, get LotR2 and you can have $10 from me)