I recommend Alter/Ego. It's free except for the French vocals. I find it about as easy as Piapro Studio, but it is much lighter on CPU usage. I find it much easier for testing out melodies because it uses MIDI and works like any other instrument. It even works with keyboard (musical or the typing kind), or any other MIDI controller. It has excellent support for vocal harmonies, and every voice bank is extremely customizable, as is typical of singing synthesis software.
The downside is that it's a bit more tedious to tune with than Piapro Studio. When looping a section of a song to listen to it over and over while tuning, I have to specify with Alter/Ego which vocals to loop, otherwise it thinks the song is continuing and uses the following lyrics. The other problem is it has timing issues, which I believe is due to using MIDI. With all other software, the note starts with the vowel, and any preceding consonant is pronounced right before the note begins, but in Alter/Ego, the note begins with the consonant, so notes beginning with consonants must be manually placed slightly early to get the timing right. Notes also last a little bit longer than the written notes suggest, so they have to be manually made slightly shorter as well.
I generally prefer Piapro Studio at the moment because I don't have to worry about manually adjusting the timing of every note according to what consonants it contains, but the problem is particularly bad for me because I make very slow music from about 5bpm to 50bpm. At higher tempos the problem is much more minor and you might not even notice it. Consonant speed can be adjusted in the program and can make the problem less severe at the cost of having more tense pronunciation. Alter/Ego is pretty much done at this point, and these issues will almost certainly not be fixed in an update, plus I don't think these problems could be fixed considering Alter/Ego is MIDI-compliant.
It's also worth noting that if you're good with any MIDI controller, you should be able to get used to Alter/Ego and time your notes well without having to mess around with manual note placement. If you're good at an instrument and like live performances, you can play any Alter/Ego VB live and do it well. Unfortunately I'm not great at keyboard, and guitar-to-MIDI software/hardware is both quite expensive and not very accurate.