Movie Themes
Some of the largest and most popular LEGO series are the film-based ones. Star Wars, Harry Potter, Prince of Persia Toy Story and Indiana Jones are just a few of the universes that have made the leap from film to brick. Each universe has a number of sets. So a basket filled with one or two larger sets, and a few of the smaller sets will make a great raffle prize.
Architecture Discovery
LEGO introduced an Architecture series in 2008 that had a sleeker and more serious look than many of the company's other collections. At an architecture event, or for a building fundraiser, there's no better way to get people interested in the raffle than with a basket filled with the Architecture Discovery series
Mixed Products
LEGO is just one of many products that people really love. There are other building toys that are also fairly popular, like K'Nex or building blocks. If you can find a LEGO set that fits with a theme like building, and toss in a set of play tools and some other building toys. Alternately, find a LEGO model that looks like whatever cause you're raising funds for the raffle, and place it in the basket with memorabilia and other things.
LEGO Games
In addition to the sets and models, there are a variety of LEGO-based board games which you can build and play. A basket filled with these games, or with one of these games among others can make a very tempting prize. A mixture of card, board and LEGO games would probably attract LEGO fans and game players alike.
Baby Basket
The Duplo system is much like the familiar plastic brick toy that all LEGO fans love, but these are designed to be larger and more durable, making them perfect for babies. A Duplo basket, or even a basket which includes Duplo in addition to other baby toys or supplies, would be perfect for a raffle where the cause is related to toddlers or newborns.
Technic Basket
For the more mechanically-minded, the Technic series gets not only to the shape and form of the model, but also to some of the inner workings. A mechanically-themed raffle would do well to include a Technic prize. The line also has a number of vehicles that could work well for car- or vehicular-themed raffles. A great grand prize for anyone who's into robotics could also be the Technic Mindstorms kit, which allows people to build fully-programmable robots.