The best toys are the ones your baby will love the most and play with often. As they get older, they'll probably go for the box rather than the toy that was once contained there, but this one will be a staple.
I recommend the Baby Einstein Link Set for its simplicity, but also educational merits. My baby girl loves the 15 colourful links which keep her amused each time she holds onto them. They are on-the-go toys which are durable and light and can be washed easily. They snap together quickly and can be attached to strollers, diaper bags, and infant carriers. The different textures allow for self-exploration or for simple lessons about colours and shapes. She even uses it to soothe her itchy gums.
They are available at Toys R Us for $11.99 and are a great investment.