Forgetting dead characters for a sec, Vegapunk, Teach, Doflamingo, Shanks, Akainu are the 5 that come to mind, they have altered the landscape of One Piece with their actions.
Vegapunk is behind every significant tech development/advancement.
Teach constantly shifts the dynamics of power and the verse (captured Ace causing a war, became shichibukai, freed lvl 6 prisoners, killed WB took over his power and territories becoming a Yonkou)
Doflamingo held the reigns over the monsters in the New World, his defeat affected how Kaidou and other nations approach future events. His past is also linked to what makes Law who he is.
Shanks is Luffy's biggest influence, you can consider him the MC or the Luffy of the middle generation. He's constantly involved in events in the background but he reacts more than he initiates (somewhat limiting his ability to impact the world). Linked to every one and their mothers (Buggy, Roger, Rayleigh, Mihawk, Teach etc.)
Akainu in a way is the 2nd half of Teach, he killed Ace (who Teach captured) changing Luffy and the SHs forever (the reason they took 2 years to train), essentially killed WB (who Teach consumed in power and status), took over as Fleet Admiral (like Teach took over as a Yonkou) moved Marine HQ in the New World making them much more active in the most important sea, than they were under Sengoku.
Honorable mention Dragon, he's probably affecting things but he's been in the background and he's been way too lowkey compared to the events that we know of regarding the other 5.
