most guns are great... for specific situations. The default gun is pretty crummy IMO because its not particularly great in any situation.
I use 3 different guns depending on what map I'm on
#1 (and the single most versatile gun): The m590 military shotgun (or its CQB counterpart). Amazing gun. Accurate while jumping, can 1 hit kill, a must have in my opinion. Great for close to medium-close range. I ALWAYS have this on switch regardless of map
#2 UMP: This SMG is my weapon of choice for close maps like Junk Flea and Death Room. Although, if the other team is camping the hallway at a bit of a distance, you will be at a significant disadvantage, as this gun is terrible at range. Regardless, it makes you fast, has a large clip, and dishes out some high damage. If I purchase this gun for a weak, all I do is play deathroom and JF. Additionally, this one of the best guns to have for CTF games.
#3 K2: This is a very versatile gun too. Its incredible accurate in single shot mode, works decent in full auto, and if you slap a scope and a silencer on it, its amazing for most medium-long ranged maps. Only downside is that it deals pretty low damage. I use this for brushwood and waverider.
Now another thing, I'm going to take a wild stab here and guess you're using a ball mouse as opposed to a laser mouse (since most people here seem to do so). Make sure you're using a laser mouse, as it will give you a huge edge.
Hope that helps some