The answer to the question is, almost tautologically, “no.” If nothing else, a mirror match is always possible, and the game does not support simultaneous turns. Someone would have to go first, and whoever didn’t would not—and therefore break the guarantee. That said, we can narrow things down such that a mirror match is required—only someone who uses the same tricks we do but more so is going to be able to go first.
As an aside, I am specifically ignoring Pun-pun and Epic Spellcasting here. Both of those are so open-ended that there’s no real saying whether or not they could overcome what we’ve got. If nothing else, Pun-pun could certainly do everything we do, and then pump initiative vastly higher than we do via reflected manipulate form, in which case the only person who could beat Pun-pun is another Pun-pun who chose to push things even further.
Anyway, limiting ourselves to slightly less nonsense than those...
Combining celerity with immunity to being flat-footed—allowing you to take immediate actions, and therefore cast celerity, no matter what—makes everything else moot. Contingent celerity does the same thing. With those in play, the person who goes first is the one who casts celerity last—which mostly entails being able to cast celerity the most number of times. Craft Contingent Spell from Complete Arcane lets you get more contingencies to have celerity in, and that is limited only by your HD, so more HD means more celerity and improves your likelihood of winning.
In the case of a tie in the number of celerity spells cast, however, the creature who initially was going to go first is the one who goes first (because the other one is forced to cast the first celerity to get the initiative). So how do we maximize the odds of that, for the edge case where we meet someone with exactly the same number of celerity spells in play?
So far as I know, there is nothing that flat-out says “you go first.” Supreme Initiative is closest to saying that, but it isn’t quite there.
The closest I am aware of to actually achieving that is the Sandstorm dire tortoise’s lightning strike (Ex) ability. That allows it to always act in the surprise round—even if there wouldn’t otherwise be a surprise round. Accordingly, it’s a popular target for polymorph or shapechange.
If you combine lightning strike with methods of preventing yourself from being surprised, then (at least on your own) no one can possibly go before you unless they, too, have lightning strike. Even Supreme Initiative fails against this. If you have allies, though, and they are surprised, then your enemies do get to join you in the surprise round, bizarrely enough. Lightning strike is not a well-written ability.
To prevent yourself from being surprised, the divine oracle’s capstone is “immune to surprise (Ex),” and just straight-up says “[the divine oracle] is never surprised.” It also applies a sane version of lightning strike, that lets you act in the surprise if there is one, but that’s not the relevant part. The goal of not being surprised here is just to not let anyone else join you in the surprise round.
At this point, the only things that can go before you are those who can apply celerity more, or those who can apply the same number of celerity and also have lightning strike. Then, and only then, does it come down to initiative. Supreme Initiative is obvious, so that only equal-or-greater-rank deities who also have Supreme Initiative can even compete, and after that, it’s just about pumping initiative as high as you can get it:
- A 3rd-level factotum’s brains over brawn (Ex) adds Intelligence (Dungeonscape)
- A 2nd-level Bayushi deceiver’s strike first, strike (Ex) last also adds Intelligence (Oriental Adventures) (requires nonlawful human)
- An 8th-level exemplar’s intellectual agility (Su) also adds Intelligence (Complete Adventurer)
- Yondalla’s Sense adds Wisdom (Races of the Wild) (requires halfling)
- A 2nd-level iaijutsu master’s lightning blade (Ex) adds Charisma (Oriental Adventures) (requires lawful)
- A 4th-level swiftblade’s arcane reflexes (Ex) adds whatever your spellcasting ability modifier is (Web article)
You could therefore spend 18 levels getting 4×Int to initiative. Nerveskitter and sign, both in Spell Compendium, are also probably worthwhile. And then just abuse +1 eager shurikens to pump things into the stratosphere, I guess. Improved Initiative isn’t terrible; the epic Superior Initiative is even better. There’s probably more you could do, but at this point your initiative bonus would be approaching +100, and you’d only need it in a fraction of cases.