The rules state:
Each dart hits a creature of your choice that you can see within range
Does this mean that it is a guaranteed hit? Or is it meaning that it attempts to hit and attempt is merely implied in all the spell descriptions?
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Yes, in Fifth Edition magic missiles hit their target(s) unerringly.
Magic Missile, in all its iterations in all different editions except for Fourth Edition pre-Essentials errata, always hits its mark as long as something isn't preventing it (such as the spell Shield, which explicitly calls the spell out as being blocked, or force resistance or other antimagic shenanigans) and is within line of sight and range. It is unique in this regard with respect to iconic D&D spells.
There is an old saying in D&D "Magic Missile Never Misses." The idea, basically, is that it's a small amount of damage (per missile) that always hits it's target.
The tipoff here is that it does not mention an attack roll or saving throw. There are other spells that do this though they are few (Power word Kill is one off the top of my head). Basically, if you have line of effect to the target, it affects them (though PWK has an exception if the target is above a certain number of HP).
So when Magic Missile says "each dart hits the creature of your choice" it really actually means that.
The only limits on a magic missile hit are those in the rules - you must be able to see it and it must be within range. It will even hit an ethereal creature providing you can see it.
If, from your perspective, the creature is hidden, obscured, invisible or out of line of sight then you are out of luck but if you can see even the tip of their nose, you can blow it off!