Here is a list, up to date as of PLZA, post-DLC, excluding legendaries, showing the Pokémon with the highest base speed stat of that type, with a non-mega option if the first Pokémon is a mega:
Normal: Mega Lopunny at 135, followed by Mega Pyroar at 125, followed by Swellow at 125
Fire: Galarian Darmanitan (Zen Mode) at 135
Water: Tied between Iron Bundle and Barraskewda at 160
Electric: Tied between Electrode and its Hisuian form at 150
Grass: Hisuian Electrode at 150
Ice: Iron Bundle at 136
Fighting: Tied between Mega Lucario Z and Pheromosa at 151
Poison: Mega Beedrill at 145, followed by Crobat at 130
Ground: Dugtrio at 120
Flying: Ninjask at 160
Psychic: Mega Alakazam at 150, followed by Mega Delfox at 134, followed by Iron Boulder at 124
Bug: Ninjask at 160
Rock Mega Aerodactyl at 150, followed by regular Aerodactyl at 130
Ghost: Mega Absol Z at 151, followed by Dragapult at 142
Dragon: Mega Garchomp Z at 151, followed by Mega Sceptile at 145, followed by Dragapult at 142
Dark: Mega Absol Z at 151, followed by Mega Greninja at 142, followed by Chien-Pao at 135
Steel: Mega Lucario Z at 151, followed by regular Mega Lucario at 112, followed by a tie between Mega Metagross, Mega Skarmory, and Alolan Dugtrio at 110
Fairy: Flutter Mane at 135
Note: this answer assumes legendary, sub-legendary, and mythical Pokémon count as "legendary", but Ultra Beasts and Paradox Pokémon do not.
Source 1: Pokédex with stats--Pokémon Database
Source 2: All Mega Evolutions Added in Legends: Z-A--Serebii (to check if any of the new megas are faster, since most of them aren't in the Pokédex With Stats, linked as my first source)
Hope I helped!
