It's all a matter of how you define serve. If a serve has to make it across the net and land in the correct court, then it's probably Nastya because then what Lena D. hits would be faults.
If you define serve as any attempt to put the ball in play, then it's Lena D. because a serve that gets over the net, no matter how much of a cream puff, is better than a serve that doesn't even hit the tape but strikes the net about a foot above the ground.
I also have to admit that I'm probably inserting the phrase "relative to the rest of their game" in my own estimation of who's serve is weakest.
Now, if anyone needs me I'll be wearing my hair shirt, flagellating myself and kow-towing before a picture of the Golden Goddess for having the effrontery to criticize her.