"best" is relative and when it comes to exhaust..seems to also be one of very personal taste, which really defines what is best.
As for flow (ie: performance), hard to beat the factory system.
As for sound, well its pretty much general consensus that dodge did it best in making them far too quiet.
cheap mod would be to drop in a mid-muffler delete or rear resonator delete, (dont do both) depending on how much drone you can live with. there are kits easily available for both delete options. a word of caution with the V6 models, they can begin to sound "tinny" (like a ricer) with the wrong one.
Full cat back get costly and to be honest, you may see no real gain in anything, flow or sound..but again, everyones ears, preference and barrier to entry is different.
best advice is to go actually listen to other cars in person, see what you like and what you dont.
What's your definition of ridiculously expensive? For full quality cat back, the Solo Performance Exhaust MACH XV-CH is around $830 . Flowmaster 817737 American Thunder is around $850. Prices climb from there
I just installed the Solo Performance Exhaust with dual round tips a few weeks ago. I can't compare it to any other systems, but I highly recommend it. It feels like a totally different car now. The exhaust sound is not annoying or "ricey" and has an aggressive note when you get on it. I installed the exhaust in conjunction with the Hellcat lower intake and headlight tube. I feel a slight increase in power and response but I have no way of knowing if this is just a placebo due to the sound of the exhaust. Solo has also released a Solo Performance V6 Challenger Street Race version of the exhaust which has an X-pipe rather than a muffler. This is supposed to be louder and less restrictive. I would have purchased this version if I did not live in California where it would be illegal. I purchased the exhaust from Solo's ebay store as ebay was having a 15% back in ebay bucks so I received a $100 voucher to use on other parts for my Challenger. I would recommend waiting for an ebay promotion and ordering it through their ebay store.Just google "solo-performanceonline ebay" to find their ebay store and search for "challenger sxt" to see all their options.