The navy seals are seen as the epitome of special operations. They are basically jr delta force in what they seem to focus on. While they come from a primary mission focus of water special operations, they also are a fully operational land unit.
Army rangers are seen as elite infantry. They specialize in direct action and raids. But many people in the rangers say they do just as much commando stuff as seals do.
Green berets are focused on unconventional warfare. Not very glamorous. Teaching third world farmers and sheep herders how to shoot and do jumping jacks. But also they are said to do just as much cool operator stuff as seals.
Delta is the tier one unit. Focuses on CT and hostage rescue. Primary focus on missions involving CQB. Seen as the elite of SOF. But then it seems that they are also tasked with things like UW. So they seem to do much more than things primarily about CQB.
Devgru is basically deltas but with a water operational capacity. So they do the ops delta would not be able to do, like getting on an oil rig or raiding a boat in the ocean. Many though say devgru is a less competent version of delta.
Based on this if you wanted to join the military and do special operations, it would make the most sense to have seals as your number one pick. As they do the more commando style missions that people find cool from movies and video games. Rangers still see action but they are more of a raid sized group. And army special forces sound cool at a glance but when you realize they prioritize unconventional warfare, they lose their cool factor. And as a deal you still can just move into devgru as a tier one mission unit, and also you could try out for delta as it is open to anyone in the military. So you have more options than someone who’s not a seal.