Starcraft 2: Wings of Liberty is my personal favorite in terms of the best all-time RTS games and it is highly regarded as one of the most popular esports games around. Featuring both a stellar single-player campaign and multi-player online experience, Starcraft 2 is a must have for anyone who likes real-time strategy games.
Command & Conquer: Generals is a real-time strategy game based on realistic warfare. Throughout the campaign and multi-player you have access to soldiers, tanks, fighter jets, helicopters, and even nuclear missiles. Stay tuned for the sequel Command & Conquer: Generals 2 which should be out in the near future!
Empire Earth is an RTS made by Rick Goodman who was the lead designer for Age of Empires. The game came out back in 2001 and is still an excellent game in today's standards. Empire has two sequels that have been primarily flops so your best bet is to stick with the original.