How can I make sure that Ana's ultimate hits the designated target
The target is locked when you press Q (or activate ultimate, for non-PC platforms).
The best way to ensure you are hitting the right target with your ultimate is down to placement and awareness. If Tracers and Lucios want to steal your boosts, position yourself so that they can't be between you and your target.
You can see from this video at 0:13, that Genji cannot deflect a nano boost, but the animation for deflecting a projectile still occurs. From this, and the instantaneous application of the boost on the target, we can infer that the ultimate is not projectile based, but has a projectile based animation to provide a visual aid.
I've played over 50 hours of Ana in competitive mode, and it took me a while to figure out how to get these ults to fire on the right target when I have more than one of my teammates in my line of sight.
- Make your teammate aware that you're looking to hit them with Nanoboost. Much of the time, they'll separate themselves from the crowd, and some will even look directly at you to get line of sight. The best case is it's someone like Genji, Winston, D.Va or Pharah who have vertical mobility and can get themselves above the crowd as a trigger for you to Nano them. This is particularly true for Genji, who gets a reset on his Dash when he pulls sword, so be explicit and ask him to dash straight up as a call for Nano. Shadowburn, one of the best pro Genjis, does this constantly.
- JUMP! Yes, just jumping will change the Nano target. I can't stress this enough: jump! This is especially effective when you have a Lucio on your team who likes to strafe around in front of you. He's short, and you'll have an easy time targeting over the top of him.
Take an elevated position. Ana works best from an elevated position, due to the hitbox size of friendlies when using her primary fire. You'll be able to see the health of and hit your wounded teammates much easier than you could from ground level. For example, Nepal: Shrine's raised walkway across from the point is an ideal location. You'll be hard-pressed to misfire your Nano from there.
Lastly, look side-to-side before you Nano. Look behind you. That Mercy might be tethered to your target and fly right through you when you hit Q. As much fun as Nanomercy is, you'd probably rather lay it on the Soldier 76.