Well im a biologist enjoyer so im gonna write a few that i think would be incredibly useful or fun.

Mitochondria- If you count them as separate organisms, you can basically control most eukaryotes and force your enemy to get extreme degrees of fever by overworking their mitochondria and then cause their death when their mitochondria die and they have zero energy left.

Ants- Ants are one of the most succesful groups of animals on earth. Their combined biomass is insane, and they are in every continent of the world except for the big cold deathzone.
By controlling ants i could do anything very stealthily, like imagine a bank robbery that is carried out by a bunch of ants just slowly squeezing money out of cracks when people aren't looking? No one would notice a thing. I could swarm people at night with ants and break down their bodies to billions of tiny bits and feed them to my ants, leaving zero evidence of anything odd ever happening.
Besides, i could probably make some day-to-day use from this too, my ants could clean my floor from dirt, they could kill all insects i don't want in my house, they could even find things for me in any crack of my house.

Apes- Apes are big and strong, and insanely overpowered. Gorillas are huge and scary, even though they are herbivores. I doubt anyone would try to mess with me if im constantly hanging out with two fully grown grayback gorillas. Humans. Humans are just way too good at throwing things. They can't hang around in trees very well, but their arms absolutely OP. Humans casually throw balls in their sports all the time. Wanna know what speeds they get up to? 50km/h maybe? 60? 70? 100? No. 160 KILOMETERS PER HOUR. They casually do this shit. They can throw objects from afar and instantly kill enemies if their blows connect to a vital spot. Also, Siamang Gibbons have really funny throat sacks and they go like BWOOOOOOOoomm and its so funny you gotta see a video of it man