Here is my list of supporting actors and actresses in television right now that I think deserve a shot at a leading role. Feel free to nominate your own favorites in the comments.
Known for: Alexys in Castle, Kara in Superman: Unbound, Melissa in We Are the Millers
Why she rocks: She was 12 years old when she got her first acting role in Camp Winoaka, and since then she has had mainly voice acting roles (especially in children's programming like the Winx Club) in addition to being a series regular in Castle. For over five years I've seen her as Alexys in Castle, and she has been amazing, not only can she easily convey intense emotions she also is great with nonverbal communication and expression. Quinn is now in the prime demographic for launching a career as a leading lady, and she deserves a chance to shine.
Known for: Captain Jack in Doctor Who, Malcom Merlyn in Arrow
Why he rocks: He can act, he can sing, he can dance, he is unstoppable. After being turned down in the role of Will in Will and Grace for acting too straight, Barrowman first won fans over by his portrayal of a sexually promiscuous pansexual in Doctor Who and then again as the devious former League of Assassins member Malcolm Merlyn in Arrow. Yes, Barrowman already was a leading actor in the show Torchwood, the Doctor Who spin off show, but Miracle Day (IMO) kind of sentenced it to a permanent limbo (because it wasn't officially cancelled) which means we won't likely see its return. Give him another chance.
Known for: Claudia in Warehouse 13
Why she rocks: Scagliotti first got her acting start in minor roles in children's television programming such as Nick's Zoey 101 and Drake & Josh. She dipped her toes into the science fiction genre with a minor role in the web based miniseries Gemini Division. Then she jumped headfirst into the wonderful (and cancelled too early) Warehouse 13 where we met the cocky and headstrong, yet adorable Claudia. She has great facial expressions and wacky gestures down to a science. Scagliotti is a fantastic comedic actress who herself is adorable and geeky, have you seen her video blogs from her Warehouse 13 days on the Sci-Fi web page?
Known for: Badger in Firefly, Romo Lampkin in Battlestar Galactica, Benedict Valda in Warehouse 13, Canton in Doctor Who, and of course, Crowley in Supernatural
Why he rocks: Because he's freaking Mark Sheppard.