Asking only about the female singers of Nightwish in this poll and your options are:

For me, as much as I would love to give my vote to the incredibly versatile Floor Jansen, Nightwish has been underusing her full range and potential, so I have to vote classic and say Tarja. 

Over time I have come to see Angels Fall First as really quite a poor album although the opener Elvenpath remains one of their best songs.

Then you have what I feel is their classic period - Oceanborn and Wishmaster. I think those two albums are bloody brilliant.

Then you get Century Child and Once which I think are also good and on a more generous day I may even include the former in the classic era, but overall I don't believe either holds a candle to their best.

Then they give Tarja that very public sacking and Tuomas seems to become all 'fuck what the fans want' and deliberately recruits a very different singer while writing an album that musically sounds like it wants Tarja's voice. They are some decent songs on Dark Passion Play yet for me the one that has most stood the test of time is the acoustic, Marco Hietala sung number The Islander. I also much prefer the instrumental version of the opening epic The Poet and the Pendulum. 

With Imaginaerum though I think they made one of their best albums. The music is more adventurous and actually feels written for Olzon's voice. I actually hated this one when it came out but over the years it grew on me to the point of being in my top three. 

Floor Jansen joining the band should have been a sign of great things to come. She's an absolutely brilliant singer and a much more logically voice to follow Tarja's legacy with them. And yet it feels like from day one the band has deliberately undersold what she can do. I like the Endless Forms Most Beautiful album but on top of coming across as vocally restrained the music also feels like Tuomas wrote it to be really 'safe' after Imaginaerum so it ultimately became a let down all around. 

I think I streamed the one that came after it only one time and never felt like doing it again. I certainly haven't bought it. I should probably check it out again before the next one drops to see if my opinion has changed at all. 

My point with all this is that a singer can be great, but sometimes a singer is only as good as the music they have to work with. Floor Jansen is bloody brilliant and versatile and should get my vote any day of the week for that reason. Just not when it comes to Nightwish. And I'm still kind of bitter that her joining Nightwish meant killing her own project Revamp which had only just begun to show it's full potential two albums in. 

Regardless, Tarja remains the most iconic Nightwish singer, had the strongest overall body of work with the band, and fits their sound by far the best, imo.