I used a hydration pack for the first time for the gravel worlds race. I chose the lezyne adroit. It's huge, but very light. Holds 100oz of water, or 3 liters. Refilling it was easy at the rest stops. It's got tons of space for tons of stuff. The best feature, are the pockets on the belt strap. I was a little worried about how I was going to grab food out of the bag while riding, but these little pockets could hold plenty. 3 clif bars, or 5 gu's, you get 2 pockets at your hips, that's easily 40 miles worth of food.
By the time I had the whole thing packed for a 150 mile ride, I still had room for a change of clothes. You can get these 1/2 off the msrp at only one place.
Having not used any other hydration pack, I don't have a lot to compare it to. I saw some dudes with big ones like mine, they didn't have many straps, and were all over the place. This thing gets tight and secure with just a few adjustments. The water stays cool thanks to material between the riders back and the reservoir too.