There are plenty of overlooked and overhated characters in DB that I can't just pick one so I apologize in advance for a long post:
1.South Kaioshin- Strongest of the Kaioshin that Buu slaughtered during his last rampage before being sealed away, you could tell he has this thirst for battle similar to the Saiyans and he was so strong that he actually put up a better fight than even SSJ3 Goku, Buu had no other choice but to absorb him in the end
2.Yamcha-If you ignore TFS and other Yamcha memes, Yamcha is not even that much of a pushover, that Saibaman caught him by complete suprise after Yamcha won the fight fair and square, not to mention, he has shown he was tough and somewhat dedicated before and after(if you include the filler) that infamous fight. Very glad SDBH is giving him attention though
3.Raditz-Same as Yamcha but in a different tone, people bash on Raditz cause he is a fly to what characters are capable of now, but you have to remember how terrifying he was in his day with his design, personality and most importantly strength that toppled two strongest warriors like they were nothing, plus he had the potential given the fact he was Goku's brother but ultimately had to get put down shortly after making him just a warning sign for the stronger foes
4.Trio de Dangers- All 3 are very unique in Dragon Ball with their abilities and gave Goku and others quite a good fight, they had room to develop due to being inseparable brothers from a cesspool of a universe and I personally am really looking forward to see more from them in the future(especially Bergamo)
5.Dr.Gero-The mastermind behind all the pain in the Cell Arc, everybody gives attention to the Android twins and Cell but not the one who spent decades preparing for the day he could avenge the very army he was working for. He is an amazing example of a mad scientist dedicated to his goals. But like most mad scientists, his own creations prove to be the death of him.