Easily Season 3, without question IMO.
We have great standalone story-telling which is enhanced by their being the climax of everything that has been developing since Season 1, plus we have some amazing dramatic moments - Earth vs Babylon 5, G'Kar's spiritual development, death of Kosh, etc.
The trouble with season 4 is that it came across as rushed - and individual story arcs such as Sheridan's desire to abandon his responsibilities (and the fate of the universe) to save his dad, only to be rescued by his officers acting like the 3 Stooges was plain insulting.
Season 5 is just "Gee, we got another season, but we don't know what to do with it..."
2c.