1911, because you get very little gun for the money. The 1911 was designed in an age when labor was cheap and unless you can get yours put together by a decent gunsmith, it's going to have reliability or accuracy problems. Modern designs offer all the same features at a cheaper price. The 1911 is a good gun and a very historically significant one, but it doesn't deserve even a 10th of it's near godlike reputation amongst gun people.
Second most overrated is H&K anything, especially the USP. Cracked slides, meh fit and finish, huge price- I'll pass. The G36 is basically an overpriced AK-47. The MP5 is an overpriced Uzi. Again, these are all decent guns, but not good in any relation to their cost or their reputation as ultimate killing weapons. Oh yeah, don't get me started on the HK-416 "The same thing Oly Arms has been doing for 15 years, only twice as expensive! And you can't have one if you're a civilian because you suck!" And the fanboys love them for this!
I guess the number three slot has to go to Glock. They are a decent gun IMO, but they deserve the number three slot because of how mind blowingly awesome people think they are and because of how I am biased towards DAO and DA/SA pistols. It is actually a good gun IMO. I keep meaning to pick up a used one.
I think number 4 goes to the M1 Garand. Hahaha, bet that pissed you off. But seriously, it doesn't deserve the label "finest battle implement ever devised"- please, give me a break. It is an overly powerful, hacked together design with way too little magazine capacity. At the very best is represents a springfield on steroids rather than a real advance in firearms technology. I love the M21 and FAL though, but they get way less attention despite being the guns that the Garand should have been.
I don't think that the AR/AK/SKS are overrated because their faults are all well known and commonly decried far in excess of their merits. The truth is that they are all very good weapon systems for the money (yeah ARs cost more, but you only have to buy a lower once).
The deagle isn't overrated because everyone knows they suck.
Most underrated gun is the Saiga-12 because almost no one knows they even exist. These are the same people that fap uncontrollably at the mere mention of the AA-12.