In the first play-through of DS I only used the Plasma cutter and it was fun at first but then it was so powerful most enemies were a breeze. now up to chapter 6 in DS2 and haven't really cared to buy other guns. Is there any recommendations for the other guns, because otherwise I'm just going to be saving up to by more types of RIG's.  
 
Why do I feel that I'll just new game plus it?

I went through the first game with basically just the plasma cutter too. I'm really liking the line gun this time though.

The Falmethrower is pretty cool, and I like the contact beam too.
 
I also like the seeker rifle, not because I think it's a particularly good gun, but just because it's satisfying to kill enemies with.

I just started a New game+ in DS, and i think the Line gun is amazing, granted, i have it fully upgraded. It devastates any- and everything it shoots at, it's great. 
 
The plasma cutter is the best starting weapon of all time though.

I went through most of the game using the pulse rife, I felt that it was a really good gun (accurate, fast rate of fire) but its not as powerful as the line gun.  

" I went through most of the game using the pulse rife, I felt that it was a really good gun (accurate, fast rate of fire) but its not as powerful as the line gun.   "

It seems like every weapon is pretty damn good when used right. It's really up to you. Pretty cool that they managed to make each one feel so different and be useful.

The Seeker Rifle is godly. It's basically just the DS equivalent of a Sniper Rifle but oh man, it's powerful and when you zoom in it has a damage bonus. On Zealot I was killing enemies in two shots without upgrading anything on it. I just finished fully upgrading it and have yet to test it out - I'm expecting epic proportions.

Have not tested all of them out myself but all weapons seem to be viable this time around.
A lot of people seem to be praising the Impaler due to the fact that when upgraded it has the DMG of the Contact Beam but the ammo for it is dirt cheap, most enemies die in 1 hit.

the one that comes with the collector's edition is pretty good or take the flamethrower thats good fun

The force gun is the best gun in the game.  You don't even have to aim with it unless you're using the alternate fire, which, by the way, is fucking devastating.  I can't understand why so few people appreciate the force gun.

Most of the guns are really good if you upgrade them, you can't go wrong. I personally prefer the Force Gun its kinda of like a cone aoe in front of you, very effective if you find yourself getting surrounded.