So what condition is the existing system in? Are the tail pipes ok?
I'd do one of two things:
1. Replace the muffler with one of your choice. There are quite a few available, from mild to wild but the choice is usually based on what sounds good to you.
2. Replace the muffler with one of your choice with a single in/dual out configuration. This requires new tailpipes and the outlets can be placed as you like them.
As far as which muffler, well, I can't pick it for ya. Some guys like Flowmaster, some like Borla, some like Magnaflow, etc... It's what sounds best to you that counts.
Me? I like Magnaflow but that's just my personal choice.
Good luck.