The best buses to draft are the electric trolly buses. They're slowish and don't stink. Every once in awhile I'll draft a morning bus home. In the winter it's sometimes nice because the bus acts as somewhat of a shield from the weather and the insane motorists.
One of my best winter rides this year however was the morning after the first snow fall. The roads were gross and the traffic was insane. It started out bad but I huddled myself behind a CAT Snowplow that was going my way at a sweet 25kph or so pace. Motorists tend to stay away from big machinery like that and in turn they stayed away from me. Those big wheels are a little creepy when your breaks aren't working 100% though.