I think Valkyrie is the one that you can say is a must-have. The others are situational either based on the map or your playstyle.
Frost used to be a must-have when she was OP (sniper shotty, C4, and traps), but now her traps are her main positive and those can be snuffed out by a good team. That isn't to say they aren't useful, of course. She's the best "trapper" between her and Kapkan, for sure.
I think Buck is great if you're a good rusher/attacker. He's a fantastic breacher. His rifles are great, his shotty can blast giant holes in the wall, and the frags make him great for clearing rooms or flushing Defenders out. If you're aggressive, he's a great pick.
Blackbeard is good for more cautious players. The first time you play him, you'll probably think you're a lot more invulnerable than you actually are. Great for peeking windows, hiding behind cover (or shield players), peeking murder holes. Overall, a fairly "seflish" Operator in that his shield mostly keeps him alive, so your utility to your team will be dependent on how well you can stay alive and contribute
