It depends….
Basically I use syllables starting with T for a note well separated from its predecessor and D for a more portato/tongued legato effect. I also vary the vowel sound to blow warm air for low notes and cool air for high notes.
So for a slow lyrical piece played portato I would use dah for low notes and doo for high ones. But currently also experimenting with deee for high notes.
For a faster or more rhythmic piece, especially if Baroque, I would follow the formula often given of T for accented notes, first beats in the bar, repeated notes and jumps and D for skips (2nds and sometimes 3rds). So tah, dah for low notes and too/doo (or tee/dee) for high. (If tah is too strong for a very low note I might also use dud for the preceeding note instead. I write all my tonguing syllables out under the staff like the lyrics to a song, practice them by singing and learn them off by heart. Now I’ve written it out it looks a bit complicated, doesn’t it?
I can’t get the hang of syllables starting with R or L and I haven’t needed double tonguing yet.
Since I am mainly self-taught this might all be wrong. And there are always exceptions, like low F on the treble which sometimes resists all tonguing, and needs an aaaah sound.