Some of the req weapons that I believe are underated to me are the scale of soirapt, river of light, kriths left hand, song of peace, and whispered truth.
Scale of Soirapt: The scale is available at req 4s and its basically an early access to bayonet weapons (still one hits without ammo). In most warzone games a lot of good stuff is availabe at req 4s that are more better and still has more gameplay appearance than the scale such as the blood of suban, whiplash, blaze of glory, pro pipe etc.
River of Light: Now I know that this weapon is pretty bad but thats only if you use the charge shot. Using the charge shot will continue to drain the gun with each shot fired. To me I absolutely find the charge shot terrible but if you just shoot the weapon normally it uses less enegry then the regular incen cannon plus it has slight tracking that many people do not know about. (Tracking is weaker compared to the heartseeker)
Kriths Left Hand: Honestly I like this sniper and its pretty great to get snapshots with plus a burst seems like a cool idea. With the weapon at 7 reqs most players would overlook the weapon and use a nornfang or a Halo 2 Beam Rifle Delta.
Song of Peace: With players at req two I hardly ever see anyone use any weapons till they hit 3. Otherwise some players would pull warthogs or carbines. To me the song of peace is a good starting weapon to req up with. While there is a better supressor that is the razors edge, it has the ability to supercombine but its extremely inconsistent to do so. I find myself to get more kills with the song then i do with the razors.
Whispered Truth: Probably one of the most overlloked mythic weapon other then the oathsworn, the whispered truth is a great precision weapon and has a faster kill time out of most precision weapons other than the barbed lance and the rain of oblivion. It only takes two burst to the head to kill that is if you don't miss. With the speed boost included with the weapon, it makes hit runs a lot easier.