There's not a universal answer to your question. If you can provide more details maybe we can help out.
What is your home airport? Where do you typically fly (Not just saying "Domestic US", but some examples of locations you expect to visit)? What do you mean by mid-level? 50k/year? More/less?
Oh, and what defines "good" for you? Miles accumulation? Award availability? Complimentary upgrades?