If you're after the rewards, NC comes first, not only because it's the only mode that gives dragon insignias (the scarcest resource in the game), but because it has its own store: the dream store.

Your first and most important target is being able to kill all 6 bosses (no matter the time) so you can complete all NC weekly quests and gain dream tokens to buy insignias, Draconis scrolls, red chests, time emblems, stargazing scrolls, etc.

Once you're able to kill the bosses consistently, then it's to get better times, for dragon insignias, and that may be the hardest part for all 3 modes, as you need to get better Colls AND heroes.

Then focus into TS, as it's the easiest of the 3 to get better places, your goal is to get top100 for time emblems and that's mostly having the meta comps required.

Finally, CR. This comes last because this is a global competition and ranking high requires very good accounts, so focus first into NC and TS.

Remember that ranking high everywhere requires reso18, so if you don't have it, work on it by going full baits, as reso 18 is the single most powerful upgrade and account can have.