I'd say you have to assume you don't have an infinite budget, othewrwise you could probably find a top-flight a lister for every role. As it is, my picks are probably pretty expensive. Still:
Sheridan - Jon Hamm. Mad Men has made him famous enough to headline, but his movie career hasn't exactly been electric, so he's probably gettable for the right price. And he'd do a great job with the character.
Delenn - Jennifer Connelly. She's done a lot of good acting in mediocre movies (Hulk, and IIRC, she wasn't awful in The Day the Earth Stood Still), and in good ones (Blood Diamond), and she's got the range for a character like Delenn. Plus, she's hot enough to still look good on a poster even under all the Minbari makeup.
Ivanova - Anne Hathaway. Probably would be an expensive get given her recent hits, but her Catwoman kicked arse, and showed she can play a tough action role like Ivanova would end up as on the big screen. Plus she's 8 years older than Jennifer Lawrence, who's way too young to be realistic as someone in Ivanova's position.
Franklin - This one is tough - JMS apparenty said that Franklin could have been of any race, but if you changed his race, the film would get accused of whitewashing. Mos Def might be interesting in this role - I remember thinking he did a good job in Dexter.
Marcus - Tom Hiddleston. Another who might be a bit pricey, but imagine it! Marcus is a fantastic blend of action, comedy and ineffable britishness - a good-guy version of Loki would be a really fun take on the character.
G'Kar, Londo and Garibaldi - No ideas, especially for the first two.