Where you work and how you get there will determine which container is best for you. BPA-free plastic containers with lids are probably the most popular since they're lightweight and rugged, making them great if you're carrying a lot on your commute or if you're biking or walking to work. They also fit well in lunch boxes with ice packs, which is essential if your office doesn't offer a fridge or you are on the go and eat lunch wherever you are.
You can also use glass mason jars, which are a must if you're concerned about eating out of plastic containers. These are fragile, so they're great if you're commuting in a car, bus, or train or if you're walking to work. You just want to make sure these stay upright.
Both containers are fairly inexpensive — less than $1 for the plastic and around $2 for the mason jars — and they're dishwasher-safe and reusable, so you can feel good about putting less waste in the landfill.