For English (and I think Japanese), HTS with the STRAIGHT vocoder can be pretty amazing. Very amazing. Licensing on both of those pieces is problematic. (Edit: I should probably mention that HTS is not easy to use. It will require some mental fortitude.)
Festival can produce very good results, but you have to get into the weeds with it and do a lot of planning. If you don't like Scheme, you're not going to like dealing with festival, and if you do like scheme, you're going to be frustrated by their particular scheme.
I've also noticed a lot of folks looking at post filtering, but that is a sea of dragons at the moment.