I am no sleep expert, but after two kids I have tried the gamut of sleep swaddles/sacks. Of course, every baby is different and what worked for my firstborn didn't necessarily work for my second, but here are my favorites. Godspeed!!!
The Halo Sleepsack Swaddle is by far my favorite for newborns. I tried several and this one stayed on better than any other and as your baby develops you can transition from arms in, to one arm out, to both arms out. You can also access the zipper from the bottom for easy diaper changing at night. I would buy a couple to have on hand!
Halo Sleepsack Swaddle here.
Both of my kids transitioned to the Merlin Magic Sleepsuit which we loved. It is perfect for little ones who don't need to be swaddled anymore but still want a cozy environment so they don't startle and wake themselves. You can start using it when they are 3 months or 12 lbs until they start to roll over.
Eventually most babies sleep in a sleep sack that helps keep them warm and move around the crib safely. There are a million versions but I love Burt's Bees - the perfect weight and fabrication at a great price point.