To all those who don't have the time to type in the entire word and all they write is MT, I'm typing it here so you can copy and paste the full beracha. No longer the need to waste your time and type the full thing!

Nusach Sefard:     Mazel tuv
Nusach Ashkenaz:     Mazel toiv
Nusach Eidos Hamizrach:     Mazal tov
Good old way:     Mazel tov
SUPER EXCITED: MAZEL TOV!!!!

Just choose whichever is appropriate for you.