1. Most smart 2. Mostly smart

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'Smart' have several meanings in hindi like:
1) chatur - "ch(a) - tu - (u)r"
2) Hoshiyaar - "ho - she - yaar"
3) samazdaar - "sa - m(a)z - the - ar"
4) Buddhimaan - "budh - dh(i) - maa - an"

Best one is to use 3 its formal and nice word
1 is use when you pointing clever person

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Most Smart - Bahut Hoshiyaar
Pronounce : B(a) - hu - (a)t Ho - she - yaar

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Mostly Smart - Sabse jyada hoshiyaar
Pronounce : Sub - say j(a)-ya-da Ho - she - yaar