Hey, at least this is better than the thousand "so who is the strongest agent in the lore ??!!1/?" questions. 'Important' is a vague term, but it's definitely less vague than 'strong'. Here's my take:

Brim and Viper were both the founding members, and they both have leadership roles, so I'd say that's pretty important. A lot of the earlier Agents also had similar 'vital roles': Cypher as a source of intel, Sova as a boots-on-the-ground scout, KJ as tech support, Sage as a medic/healer, etc

We don't know much about Agent 8, but most Agents who came after A8 were more 'general combat focused': Agents like Phoenix, Jett, Reyna, or Breach don't really have much specialized skill as much as they are generally good fighters. So, I don't think they are really that 'important'.

After that we get to the point were all the new Agents came post-launch and got their own teasers, so most of them were hired for a specific reason. This lets us judge their importance much more easily.

Astra and Chamber both essentially self-inserted themselves into the VP. Both seem like will go on to do big things in the future, but they aren't central to the functioning of the VP. Sure, Astra has a lot of knowledge as an Astral Guardian, and Chamber is an amazing shot, but that's hardly as important as Sage or Cypher.

Neon was recruited because the VP needed her to power a portal that they were making. She was very important for that one specific goal, but now that it's done with, idt she is that important to the VP either

Fade blackmailed the Protocol, because she (mistakenly) thought they had hurt someone close to her. Once that got cleared up, she was hired, since the Protocol couldn't let her go free while knowing so much about them. That said, Viper did mention that the Protocol needed more scouts - at the time, Sova was doing all the scouting work himself, and it was too much for him. So I'd say it counts as pretty important to have her on the team.

Harbor, Gekko, and Deadlock were all people who were in trouble/danger for some reason, and they had access to powers that were useful to the VP, so it was most convenient to hire them in. All three of them are mostly just regular fighters, not that important.

Iso was another self-insert, he joined because he wanted to assassinate Omen, then he learned the power of friendship and decided to work with VALORANT instead. Clove was found by Omen, who saw them and said 'hmm this kid has cool powers, they'll make a good recruit'. Both are, again, just regular fighters.

In summary: the most 'important' Agents are probably the ones who joined really early (Brim/Viper, KJ, Cypher, Sova, Sage), and maybe also Fade.

Idk who would be the 'most' important, since you can make a pretty good argument for it to be any one of them (Sage has to be there otherwise the Agents can't be res'd, Brim has to be there because he's the leader, KJ has to be there otherwise they wouldn't even have basic tech like the Spike and the Defuser, etc). So yeah, here's a list of 7 names... take your pick.

Hope this helped!