Pre-cap (by order of replies to question)
Maliwan Hellfire (Orange)
Maliwan Volcano (Orange)
Maliwan Defiler (Orange)
Vladof Chopper (purple)
Double Anarchy (Purple)
Well, Borderlands, With an unprecedented over 100,000 different types of guns (As stated by IGN Weekly), it would be very hard to say. I don't believe there 'is' such a thing as a 'best' type of gun. It all depends on your style, and character preference. =]
On the other hand many people have opinions on this. If you're looking for the most popular and mainstream weapons then look no further than Maliwan. Here are few which almost everyone uses:
Maliwan Hellfire (x4 Fire SMG) - Comes w/ DoT (Damage over Time), Chance to inflict Splash Damage
Maliwan Volcano (x4 Fire Sniper) - Comes w/ DoT, Chance to inflict Splash Damage
Maliwan Defiler (x4 Corrosive Revolver) - Comes w/ DoT, Chance to inflict Splash Damage, Chance for targets hit to infect others.
There are many other useful weapons, however these are the most popular. Many people believe that the Hellfire is the best gun in the game. It does have the most potential for damage of any stock (non modified) gun in the game. For this reason I believe the gun in completely overpowered, which is evidenced by characters who don't even use SMGs equipping hellfires for hard battles because it can out perform the best of the guns that they do use.
The most powerful gun in the game is The Chopper. Fires four projectiles every shot, has high damage, the highest magazine size, the highest fire rate (18) and fires the entire clip until you press the reload button. That will ANNIHILATE anything that stands in your way! Although its only available in the Secret Armoury DLC
Though only of purple quality, a good Double Anarchy puts out astounding dps. It fires 4 shots for the price of 2 on each trigger pull. and as an smg is capable of high firerate and good clip size. On bosses like knoxx and crawmerax it does the work for you.