So you want to be a Summoner?
The Deadly Spheres have the same base damage as the Sharknadoes. However, the difference is that the Spheres have a lesser range and attack by ramming themselves through enemies, whereas the Sharknadoes can detect and kill enemies from further away with their thrown Sharkrons. The Sharknadoes can phase through blocks and the tornadoes themselves deal damage to enemies and critters, but the Spheres struggle to catch up when there are many walls in the way. Because the Deadly Sphere Staff is easier to obtain than the Tempest Staff, I would use it (or the Xeno Staff if you've been lucky) on Duke Fishron. But you already have these, so...
The Xeno Staff's UFOs are excellent for any high-speed enemy because of their teleport and zap attacks that have amazing accuracy. They completely melt single-entity bosses like the Lunatic Cultist, while the Spheres and Sharknadoes better deal with bosses with several parts such as the Destroyer and Skeletron Prime. The problem with the Spheres is that their attacks are a piercing type, which give enemies about 1/6 of a second of invulnerability to other piercing attacks. It's not too noticeable, but because the UFOs have a single-hit attack, you'll notice the huge cloud of damage numbers above the enemy. Also, do note that the UFOs need some headroom above the enemy to attack properly, otherwise they may attempt to teleport but then come back because of the insufficient space. This can happen in narrow passages, or in the corners of caves and houses. The Spheres and Sharknadoes do slightly better here, though mostly the latter because of the Spheres' tendency to be derpy when it comes to wall-hugging.
Of course, don't forget Tiki and Spooky Armor, as well as the many summoning accessories to accommodate them. The Witch Doctor sells the Hercules Beetle and Pygmy Necklace if you have a Pygmy Staff with you. The best time to shop is during the night when he's living in a Jungle biome. You'll also need the Necromantic Scroll, two if you want more summoning power, and combine one with the Hercules Beetle for the Papyrus Scarab. That involves taking down a lot of Mourning Woods in the Pumpkin Moon though, so that may take a while and is a whole other topic on its own in terms of preparation. This event is the only source of Spooky Wood, which you need for the armor. Along the way you might get a Raven Staff from one of the Pumpkings. It's nice to have, but unfortunately it falls short in terms of ability when compared to the staves you already have. Spooky Armor gives a higher damage boost than the Tiki Armor, but it grants one less minion which is kind of a bummer. So what you'll want to do is equip the Spooky Armor, summon whatever you can with the damage boosts from that and certain accessories, and then switch into Tiki Armor and summon the extra minion, then change the accessories into the defensive set. I tend to have two of each accessory; one for damage, one for defense. Since damage is set on a minion only upon summoning, you're able to switch into other accessories that provide more defense as long as the minion remains out.
When we come to the final minions at the Stardust Pillar, we have something that's quite different from the rest; the Cells and the Dragon. The Cells have an excellent DPS value on both separated hordes of enemies as well as bosses, because of their ability to inflict very high damage with mini-cells as well as a stackable damage over time. This is just barely slower than the UFOs, and some people would prefer the UFOs either way, but with the right situation and setup, the Cells can be far more efficient, especially when in those places where the UFOs can't reach the enemy. The Cells' base damage is the highest of all the minions that follow you around, and their attack hits a single enemy, allowing for no invincibility frames for the bosses. The Dragon's base damage would seem rather low, but it increases tremendously when you summon more segments. Filling all your minion slots with a large Stardust Dragon can allow for the highest DPS a Summoner can get. My Dragon can easily reach between 600 and 1,000 damage with the correct setup. Because it flies into enemy hitboxes and hugs them constantly, the damage is rapid and adds up very quickly. If other enemies are in the way, they'll get shredded as well. This makes the Dragon the best minion to take on the Moon Lord with. Its attacks pierce infinitely, but it doesn't matter because the damage is so darn high that it will demolish anything before you can even start fighting it. But that's only if there's very few enemies to deal with; the Dragon's weaknesses are that it can only target one enemy at a time (which can be unhelpful if it doesn't go after the enemy you're fighting), and that it sometimes has trouble catching up to fast monsters like Duke Fishron. The enemy's speed is usually completely nullified by the teleporting ability of the Cells and UFOs. However, in the right battles, the Stardust Dragon can be considered the ultimate minion of them all.
TLDR: The minions you have are more efficient in certain places than others, and you'll want to change up your minion loadout depending on the task at hand. It's mostly up to you.
Then we have the stationary minions...