When picking out a hydration pack, one of the first things you'll want to think about is the size you need. If you're exercising at the gym or doing training runs, a smaller pack that holds around 2 liters of water will be sufficient. If you're planning a long distance race, on the other hand, you'll want a pack that holds closer to 3 liters. The best hydration packs for running will help keep you hydrated without adding unnecessary bulk.
Generally speaking, the amount of water runners should drink is about 5 to 12 ounces every 15 to 20 minutes. Erring conservatively, that means a 2-liter pack will last you about 1.5 hours. For most training sessions and short races, this will be more than enough. However, if you're planning anything longer than a 10K, you'll want a bigger pack (or just make sure there are plenty of water stations along the way).
Once you've figured out the size you need, you'll want to think about comfort. Look for features like back-padding and adjustable straps, and also make sure it isn't too heavy when empty. As for the bladder itself, it should be easy to clean to avoid bacteria growth. And lastly, it should be easy to use, with a leak-proof mouth piece that provides a steady stream of water.
I've picked out some of the best hydration packs for running and arranged them below according to their capacity, so you can easily find the one that's right for you.