Maximals:
Optimus Primal: Sam Niell

This guy just reminds me of Optimus, he's fatherly but at the same time he can get ill and all bleh.
Rhinox: Brenden Fraiser

Can't ya just picture Rhinox in a fight with some mummies? xP
Cheetor: Robert Pattinson

The guy from Harry Potter and Twilight. Hey, he's fast, right? =P
Rattrap: Keanu Reeves

He's awesome and, to me, sorta looks like him in a way.
Dinobot: Jeff Goldbloom

This one isn't a joke lol Jeff Goldbloom would make an awesome Dinobot, me thinks ^_^
Tigatron: Vin Diesel

Airazor: Beyonce

Silverbolt: Orlando Bloom

This was the only person I could think of other than Brandon Lee.
Blackarachnia: Constance Rudert

Helena Bonham

Constance from the group BlutEngel made me think of Beast Machines Blackarachnia but Helena Bonham just has Blackarachnia written all over her.
Nightscream: Bill Kaulitz

Botanica: Uma Therman

Depth Charge: Dwayne Johnson 'The Rock'

Predicons:
Megatron: Dr. Evil!

Laurence Mason:

Quickstrike: Kid Rock

When I look at Kid Rock, I think of Quickstrike ^^
Waspinator: Carrot Top xD

Terrorsaur: Jeff Hardy

Inferno: Johnny Depp

Scorpanock: Jack Black

Tarantulas: The guy that played Victor in Underworld, Philip in Shaun of the Dead, and Davy Jones in POTC2/3
