Well, I sure there's people on here and elsewhere that can espouse the wonders of Blue Seed better than me. So I'll take Hellsing, which is running into three continuities at last check, TV (which that set is), OAV & manga. Starting with the TV's probably the best way to get a taste and go from there, as it uses early chapters of the manga for influence and is it's own self-contained story. The OAV apparently ties more into the manga as by the time they got around to making them more chapters of the manga had been done - kinda like OMG! (just more bloody)- so the OAV animates the kick-arse awesome sequences of the manga and features Nazi Vampires (something to upset the German Government.)
--Rod.H
--Rod.H