There are several sub-genres to the RTS genre itself, and since you didn't specify which type you are looking for, I will give several sub-genre lists with a few picks in each, while also assuming you are only speaking of PC RTS games. If you did just mean Starcraft's form of RTS, I am sorry for my misunderstanding, but I encourage you to read the rest of the list to see if there's a game that intrigues you. I will also give a link to the Metacritic page of each game so you can read different site reviews to get a deeper understanding of each game.
Real Time Base Building- Most common, and largest RTS type
Command and Conquer 3 -
Though 4 is out, I would suggest buying 3. Not only is 3 chaper, it also includes a more traditional style of C&C gameplay. However, you can read further into C&C4 to see if the tweaks they have made to the formula appeals to you. As for the gameplay, C&C3 is the standard RTS formula, so don't expect too many surprises.
Command & Conquer 3: Tiberium Wars
Command & Conquer 4: Tiberium Twilight
Supreme Commander -
Again, there is a sequel out, but the original is far superior. Maps are gigantic, sometimes spanning 81x81 km. Battles can be long, but base assaults are fought with a count of hundreds, and sometimes thousands of units. There are 3 tech levels for you to progress through, that includes Tech 3 for both units and buildings. At the top tech tier there is an Experimental class that includes the most powerful, with each experimental unit/building being unique in it's own right.
*My choice for this sub-genre*
Supreme Commander
Starcraft 2 -
You should know how Starcraft works, so I won't go into much detail here. I have not yet picked this up, but the story mode is strong enough to tempt me into buying SC2 for that feature alone. There is no monthly fee, as Klown said.
Starcraft 2
Warhammer 40K: Dawn of War 2 (revenge of the long titles) -
Dawn of War 2 is a more personal type of RTS than others. The single player campaign has you commanding 4 squads, yes, only 4, throughout missions, collecting items to equip to your squads all the while. There is a mode known as Last Stand, where you and 2 other players each choose a race Commander unit and survive 20 waves of enemies, where each commander has special abilities and unlocks different items through the progressive leveling system. The multiplayer is also scaled back from the previous DoW game to include no base building (units are created by a single building), Force Commanders who can use special abilities, and less units per player, making the player use a more tactical approach than "create unit blob, move unit blob" style of gameplay. Units can also get behind cover to gain defense bonuses, a feature started by Company of Heroes, which segways me to my next game.
Warhammer 40K: Dawn of War 2
Company of Heroes -
Similar in several ways to Dawn of War 2. Both DoW2 and CoH rely on capture points to give resources to the player while units can take cover behind objects for defense bonuses. Almost all of the battlefield is destructible; tanks can plow through walls and some buildings, bombing runs can clear whole hedgerows, and artillery can level complete areas.Company of Heroes does include base building, which makes Company of Heroes a comfortable middle-ground between DoW2 and Starcraft. In multiplayer, CoH also lets the player choose which of 3 style of Support Upgrades they will use, which varies for each faction.
Company of Heroes
Turn Based RTS-[size=11]I know it's not considered "real time", but should not be excluded[/h]
Civilization 4 or 5 -
The Civ series is the most well known Turn Based RTS, and the distinctions between 4 and 5 are heavy enough to really create 2 very different, but still similar games. Civ4 is a much more complex game than 5, which is up to the user to decide if this is a good or bad trait. Civ5 streamlines many features, removes many annoying problems throughout the series, and adds several much needed tweaks. Read the reviews to decided which game suits your taste.
*My choice for this genre*
Civilization IV
Civilization V
Empire: Total War -
The Total War games have always been rather odd in their classification. On the overworld it is a standard turn based rts by building an empire and managing the empire, but once in combat, Total War games become a RTS game that has you controlling your units around a battlefield to defeat the enemy's units. There is no base building or resource gathering in battles, so in Total War games you must rely heavily on your strategy to win, and not always on force girth. Empire: Total War takes place during the 18th century and includes and, for the first time in a Total War game includes naval combat similar in style to the land combat (previously, naval combat was fought based on statistics where the player had no interaction).
Empire: Total War
Europa Universalis III -
To be frankly honest, I have never actually played Europa Universalis III, with the only experience I've had with the series being a 10 minute confusion fest with the original game (you cannot grasp the gameplay in under an hour), so all I would really know about EU3 would be from the reviews I read. However, EU3 is a simplified entry into the series and most "grand strategy" RTS fans will tell you to buy this. Be prepared, though, this series is one hell of a thing to wrap your head around. But with great depth comes great rewards.
Europa Universalis III
Other- For what I can't classify
World in Conflict -
What I would consider a cross between a standard RTS and Dawn of War 2's campaign, World in Conflict has you controlling a few units at a time. Whenever a unit is killed you have Reinforcement Points, which replenish over time, you use to call in more. It's an incredibly simplified, but incredibly entertaining way to approach the RTS genre and warrants a demo play if nothing else. As you attack enemy units, capture points around the maps, and be generally useful, you gain Command Points that let you call in bombing runs, artillery fire, and other support features.
*My choice for this genre*
World in Conflict
Sins of a Solar Empire -
Perhaps I should include Sins (of a Solar Empire) in the original group, but the gameplay feels different enough to be classified differently in my opinion. Sins is a space combat game where you build structures, build units, and capture worlds, similar to other RTS games. Like Supreme Commander, the scale of Sins is mind-boggling, having a map where there can be 10+ Solar Systems. Where Sins differs in gameplay is that each world has a "gravity well" that ships must warp in and out of. This twist adds much more than you would expect, and really sets Sins apart from games like Supreme Commander and Starcraft. There are also several features included from other sub-genres, such as an economic Upgrade Tree (military, economy, and unit cap) and diplomacy between players.
Sins of a Solar Empire