Like others I applied to Sofi. The application process was pretty painless, save for getting my loan information from Navient (they are a pain to work with). However, I was not happy with the interest rate they quoted me given my financial picture and credit history.
I ended up turning to Earnest. The application process is a bit more rigorous as they ask for a lot more information. This could definitely turn someone off, seeing that you have to link all of your financial accounts (bank, credit card, loan, etc.) when completing the application. I just changed my account passwords after approval to be safe. That being said, the application process didn't take long at all. Given that they take a broader look at your financial picture, they are able to provide you with what I would consider a "fair" rate. I really liked the fact that you could tailor your repayment terms based on a sliding scale (i.e. the larger the payment you chose, the smaller the interest rate and term of the loan). All in all, Earnest has my vote but I'm biased.... probably a bit salty at the rate Sofi gave me as well :)
As someone else mentioned though, apply to all lenders for refinancing. It doesn't hurt to throw a line out and see what they come back with. Best of luck to you!