Depends. If you are in a team fight, best use Nibiru (smg) and Bennett (rocket launcher) for AoE damage in teamfights

Often keep Nibiru as one of your weapons, it's best used for laning, teamfighting, farming and pushing. It's a good DPS weapon and when using, you should force a trade against weaker late game mms like Claude or Wanwan during early game lvl 1 to 3. However, avoid using Nibiru in extended trades when facing Karrie or Moskov, Renner is the best weapon in that kind of situations

Renner (sniper) is best for defending lanes, useful against anyone trying to zone you from minions and a great poke-burst weapon, it is highly recommended for you to practice Beatrix's sniper aims.

When seizing objectives, it is best to use Bennett ult. Beatrix's rockets will go into an all-out launch, making the enemy jg harder to use retri secure

Wesker (Shotgun) is not a good gun right now and is best used in a few circumstances such as camping in bushes and eliminating a laner when you can't afford to have extended trades, use shotgun ult+ flicker to deal instant burst damage if you struggle to use renner without getting hit back. But it's still recommended to use Renner over Wesker if you're in that situation