principally

chiefly, mainly, essentially, substantially, materially, eminently, pre-eminently, superlatively, supremely, vitally, especially, particularly, peculiarly, notably, importantly, fundamentally, dominantly, predominantly, basically, largely, first and foremost, in large measure, first of all, to a great degree, prevailingly, prevalently, generally, universally, mostly, above all, cardinally, in the first place, to crown all, for the most part, for the greatest part, before anything else, in the main.

Antonyms slightly*, somewhat*, tolerably.