I figure I ought to answer my own questions, so here we go.
Who is/are your favorite character(s)?
1) Londo; 2) Marcus; 3) Delenn; 4) Sheridan; 5) Talia (with Ivanova/G'Kar [tie], Lochley, Vir, Kosh, and Garibaldi/Lyta [tie] rounding out my Top 10)
What is/are your favorite season(s) of the series?
Season 5 (as I already said earlier) (although the entire series is awesome as a whole, I really liked S5)
What is/are your favorite episode(s) of the series?
Born to the Purple (S1)
Mind War (S1)
Bayblon Squared (S1)
The Coming of Shadows (S2)
And Now For a Word (S2)
Passing through Gethsemane (S3)
A Late Delivery from Avalon (S3)
War without End (S3)
Whatever Happened to Mr. Garibaldi? (S4)
Racing Mars (S4)
The Deconstruction of Falling Stars (S4)
The Very Long Night of Londo Mollari (S5)
Day of the Dead (S5)
The Fall of Centauri Prime (S5)
What are your favorite quotes from the series (one or two from each season)?
I'm partial to the majority of the quotes that are present in the S5 opening, although I also like Delenn's rebuke of the scholars in The Deconstruction of Falling Stars.
What is the one story arc or plot thread you would have liked to have had more information on?
I would've liked to have seen more of what happened to and with G'Kar and Lyta, specifically what happened to cause G'Kar to lose his prosthetic eye replacement (since, by the time he and Londo strangle each other in 2278, he's wearing a bandage over his face again) and what role Lyta played in the Telepath War. I also would've liked to have seen what happened to Captain Lochley and, as I said, wish that JMS would have at least mentioned her fate in Sleeping in Light.