It's probably Grass by four reasons
1st: You lose your Ground, Water and Rock disadvantage.
2nd: If you want a Ceruledge in a sun team, it would be handy having the move Solar Blade, which is fully charged in the sun and you would get a Tera STAB.
3rd: If you actually use Ceruledge in a sun team, you won't need to worry about your Fire weakness, because of Flash Fire.
4th: Even if Grass has a lot of weaknesses (5), you just wouldn't Terastalize when facing a bad matchup, because Fire/Ghost and Grass don't share any weakness.
(And you are also immune to powders and spores)
If you want a moveset for Ceruledge, here is one for a Tera Grass in a sun team
Ceruledge@Aguav Berry

EVs 252Atk / 4SpD / 252Spe
Adamant Nature
Tera Type: Grass
Swords Dance
Solar Blade
Bitter Blade
Night Slash/Shadow Sneak/Psycho Cut
Hope it helps!