Ultimately it comes down to your playstyle.
Are you more of a player who likes to roam when defending? In that case, I'd agree with your friends that Caviera is a good and strong pick. Ela is another operator who is really strong and highly picked.
Meanwhile, if you are more of a player who likes to bunker down in a room and defend, then someone like Mira may be a good pick.
If you are a player who usually plays more of a support role for your team, then playing as Valkyrie can be useful, as you can help provide extra vision for your team.
Honestly, they are all good in their own way, you just need to figure out which fits your style.
On a personal note, my favorite DLC defender is Frost. Love the idea of the traps that can be placed in strategic places to instantly take down people, and I enjoy her gun.
Hope this helped!
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