1. Discontinued LEGO sets

LEGO set’s value tend to increases over time. The value usually spikes up after it gets discontinued.

An example would be the LEGO City 4439 – Heavy-Lift Helicopter. This set was introduced in 2012 (MSRP: $49.99) and got discontinued in 2014. The latest market value for this set is $60 (January 2015) and is trending up.
Another example is the LEGO DC Super Heroes 6860 – The Batcave (MSRP: $69.99). The price is climbing up on eBay. Latest market value (January 2015) is $84.

The tricky part is to know when a set will be retired.

Retailers like Walmart, ToysRUs and Target often give deep discount on LEGO sets that are about to be discontinued. Watch out for those LEGO clearance.
The official LEGO website also has a section that list the LEGO sets that are retiring soon (http://shop.lego.com/en-US/Retiring-Soon).

2. Seasonal Sets

Christmas time is not only the best time to buy LEGO sets because of the typical annual holiday sales, it’s only the best time to collect those seasonal sets.

The popular 2014 LEGO 10245 Santa’s Workshop (MSRP: $69.99) was one of the top sellers during the 2014 holiday season. It was sold as high as $160.

The seasonal sets values tend to go down after its season is over and will come back up in the following year.

The LEGO 40120 – Valentine’s Day Dinner (MSRP: $9.99) is currently selling at $17 price range.

3. UCS Sets

The Ultimate Collector Series is a safe LEGO investment bet. The classic example for it is obviously the massive LEGO 10179 – Ultimate Collector’s Millenium Falcon. This set was released in 2007 with an MSRP of $499.99. Its market value now is $3,200 which is basically over +500% investment gain.

The downside of these UCS sets are their high MSRPs.

4. Modular Building Sets

The popular LEGO modular sets are proven against the test of time. Their values are almost guaranteed to multiply over time. LEGO 10182 Cafe Corner, which is the first LEGO modular building set, is currently selling at $1,500 on ebay (+930%).

5. Star Wars Sets

Star Wars sets are usually hit and miss. Keep an eye on large sets because they typically are more valuable over time, especially those iconic ships like LEGO 10143 – Death Star II or LEGO 10030 Imperial Star Destroyer.

Some of the Star Wars sets also have cool and unique mini figures. These mini figures can be sold separately on eBay.

5. DC Super Heroes and Marvel Sets

DC Super Heroes and Marvel Sets have cool and collectible mini figures. These mini figures can worth a lot of money especially when you have a complete set, such as Avengers (Hawkeye, Iron Man, Hulk, Captain America, Black Widow and Nick Fury) or Guardians of the Galaxy (Groot, Rocket Raccoon, Star-Lord, Gamora and Drex). Avoid those sets with generic vehicles or buildings that won’t have any values without the mini figures.

Check out the BrickInvesting Most Valuable LEGO Sets page for a complete list of most valuable LEGO sets of all time.

LEGO 76025 – Green Lantern vs. Sinestro Cage instruction.

The Wookiee Gunship set is a 2015 Star Wars Rebels set that contains four mini figures.

Wullffwarro

Wookies

Kanan Jarrus

Kanan Jarrus in this set has brown hair, unlike the Ghost Ship set that has black hair.

This interactive LEGO timeline chart shows LEGO theme releases since year 2000.

LEGO Star Wars, Technic, Advanced Models and Duplo have been released every year since 2000. The popular LEGO City theme was introduced in 2005 and has also been released every year since then. LEGO Chima and Minecraft were released in 2013 and are still going strong.

LEGO did make some experiments that weren’t as successful. For example, LEGO Mickey Mouse only existed for one year (2000), LEGO Spybotics (2002) and LEGO Atlantis (2010-2011).

There are also “seasonal” themes that coincide with movie release in that particular year, such as LEGO Avatar (2006), Cars (2011-2012) and the new Jurassic World set (2015).

Another notable theme is the LEGO Mindstorms that was first released in 2011. This theme has not received any new releases since 2013.

The girls oriented LEGO Friends theme was introduced in 2012 and LEGO seems to be committed to it by releasing new sets every year.