Really / very
The intensity dial: five words that each go before the same adjective and set how strong it is.
sugoku
really; very
すごく
The everyday spoken 'very'. It goes before an adjective or verb and works in both casual and polite speech.
kore, sugoku oishii
This is really good.
これ、すごくおいしい
kore, sugoku oishii desu
This is really good.
これ、すごくおいしいです
Casual speech often uses sugoi as the adverb (sugoi oishii). Natives say it, but it counts as sloppy and slangy: recognise it, produce sugoku, and never use sugoi + adjective in desu/masu speech.
honto ni
polite hontou nireally; truly
ほんとに 本当に
Adds 'really / truly' to an adjective, verb or thanks (honto ni arigatou). It stresses sincerity more than size.
kore, honto ni oishii
This is really, really good.
これ、ほんとにおいしい
kore, hontou ni oishii desu
This is really delicious.
これ、本当においしいです
The bare reaction 'honto?' (really?) is a separate item in G.
kekkou
pretty; quite
結構 けっこう
Means 'pretty / quite', and signals that the degree is higher than you expected.
kore, kekkou oishii
This is actually pretty good.
これ、結構おいしい
asoko no mise, kekkou oishii desu yo
That place is actually pretty good, you know.
あそこの店、結構おいしいですよ
Trap: 'kekkou desu' on its own (often with a small hand wave) is a polite 'no thanks' (see Responses, reactions, fillers & social phrases), so never use it to accept an offer; say 'onegai shimasu'. 'Kore de kekkou desu' means 'this will do'. Before an adjective (kekkou oishii) it never means 'no', but never say kekkou oishii to the person who made or chose the food: it implies you expected worse. Use sugoku or hontou ni instead.
kanari
quite; considerably
かなり
A strong but measured 'considerably'. It sits between kekkou and sugoku and sounds factual rather than excited.
kore, kanari oishii
This is seriously good.
これ、かなりおいしい
kore, kanari oishii desu
This is seriously good.
これ、かなりおいしいです
metcha
polite totemosuper; really
めっちゃ とても
Casual 'super / so', the strongest everyday intensifier among younger speakers. Switch to totemo or sugoku in polite speech.
kore, metcha oishii
This is super good!
これ、めっちゃおいしい
kore, totemo oishii desu
This is very good.
これ、とてもおいしいです
Kansai in origin, now used nationwide. vocab-trainer spells it meccha. The polite pair totemo sounds stiff among friends (use sugoku there), and with a negative it means 'no way': totemo muri = can't possibly.