I could approach this 1 of 2 ways. Either realistically, or comedic.... Let's go realistic first:
Luke: I probably agree with Zack Efron here as well (or possibly Orlando Bloom? Dunno)
Han: Jensen Ackles (for those who know Supernatural)
Leia: Keira Knightley (she already played a Padme look-a-like in Ep I. So sure, why not)
Tarkin: Christopher Lee
Obi-Wan: Bill Nigh (played Viktor in Underworld)
Chewbacca: Peter Mayhew (don't know anyone else)
Darth Vader: Voice: James Earl Jones. Portrayal: Till Lindmann (lead singer for Rammstein. Tall, scary looking guy. lol)
Lando: Billy Dee (why ruin a perfectly good character? lol)
Now, on to the comedy version (with a few more roles thrown in because I have thought about this at length
)
Luke: Simon Pegg (Shawn of the Dead and Hot Fuzz)
Han: Bruce Willis (Hans Solo as played by John McClaine as played by Bruce Willis. lol)
Obi-Wan: Oded Fehr (Resident Evil: Apocalypse. Played Carlos Olivera)
Lando: Martin Lawrence (Blue Streak and Bad Boys)
General Veers: Robin Williams
Admiral Piett: Stephen Colbert (with his side-kick Officer #12 as played by Jon Stewart)
Emperor: Al Pacino
Darth Vader: Adam Sandler (both portrayal and voice)
Tarkin: Christopher Walkin
R2-D2: Voiced by Stephen Hawking
Yoda: Jackie Chan
Darth Maul: Jason Statham (Transporter and Crank)
All Battle Droids: Gilbert Gottfried (if anyone remembers it, he was the voice of the parrot in Aladdin. lol)