There are a few old threads that ask this same question from time to time. A few pre-questions you need to consider:
1. What is the scope of your operation; if simple CONUS point to point you can flight plan very simply.
2. Do you have any special requirements like tanking or firebombing? (never know!)
Otherwise if your needs are basic I can advise:
a. If you have the budget Jetplanner is probably the leader. Its strengths are its accuracy and features; its weaknesses are - IMO - lots of buttons to fiddle with, weak crew interface (unless its improved), and the customer interface is 'okay' but not great.
b. I've used PPS for years and it's pretty good. It's strengths are its simplicity compared to Jepp and it has a nice DIY crew interface; it's weaknesses are I don't ever quite seem to be able to get the performance that Jepp seems to get - case in point being a Global 5000 profile that we just can't quite seem to get to tweak to get performance equivalent to a Jepp version. That said, there are fewer PPS staff and so you normally get small company interest. (In the case of the Global above the user can tweak the aircraft profile manually, albeit its a dark art, but Jepp don't let you...as far as I know).
There are others on the market; I hear good things about Rocket Route and ARINC were bringing something onto the market a while ago - it's biggest issue when I saw it in 2013 was they hadn't given much thought to non-US operations.
my 2c