@spongebob for these sort of questions, they're usually checking two things:
1. If you've applied to firms that are similar to them
2. How dedicated you are to their firm in particular
The reason being, if you apply to firms that aren't necessarily this firms competitors, it could signal that your interest in their offering isn't particularly strong enough and you may leave upon qualification. Instead, they want to see that they are your top choice (as you said) so there is no risk of wasting investment here.
The way to approach is therefore, focus in on 2-3 firms that are very similar that you have applied for. Either similar is international reach, practice area strength, think back to your reasons for applying to this firm and try to get them to align with that (even if it is very minimal). They may want to know why it is you applied for them, so be sure to keep your interest in law consistent here. As for setting them as the top contender, I'd hope they don't ask directly (that feels a little gauche) but even if they do/don't - you should still naturally end with this, by honing in on what makes them better or why they interest you more. It helps to be as specific as possible here, so if talking about a case they worked on, or the types of clients the engage with helps then reference that. Their services/client base will differ slightly.