RED RANGER/Matthew Gray Gubler
I think Gubler could give an interesting take on a character who was played as an athlete. He can play him as a more modern leader with wit and intellect similar to his character on Criminal Minds.
GREEN RANGER/Matt Smith
Again, it would be a new take on an otherwise jocky character. He can be a co-leader and equal Gubler's wit.
PINK LEADER/Milana Vayntrub
She is a very funny unknown actress who as of now is best known for her AT&T connercials. I feel having seen some of her works she could play the character as a funnier woman.
BLUE RANGER/Dane Dehaan
Dehaan is an incredibly talented actor and could play Blue very similarly to how he was on the show, while adding a newer twist.
YELLOW RANGER/Jamie Chung
She's very talented, and has gained notoriety in recent years. She proves she can take control of a situation as Stu's wife in the last 2 hangover films.
BLACK RANGER/Donald Glover
Donald Glover has been up for Spider-Man for years now, I think this would be a different choice, as he can play a funny, yet serious character in preparation for Spider-Man.
Anthony Hopkins has a very powerful voice, he can easily portray a character who you only see the face of, and has an echoing voice.
ALPHA 5/Jim Parsons
Sheldon Cooper is pretty much a robot so it wouldn't be much of a stretch.
LORD ZEDD/Michael C. Hall
Michael C. Hall is an incredibly intense person on screen, that would translate greatly into Lord Zedd.
RITA REPULSA/Rachel Weisz
She is a talented actress known for playing a very badass undead fighter, she could translate the badassness into a villainous character and still be great.
Feel Free to leave your own choices in comments.