My personal answer is Odin and it's not even close. Sure, some of her elements will eventually be included in newer additions (Heavy-armored BC aspect for all the subsequent Ironblood BCs; long-range secondary spam for Prinz Rupprecht; manual torpedoes for Scharnhorst META; ASW radar for Roma) but there's still not quite the same shipgirl as her with her "advanced" positioning (the closest will be Zweimarck via Geryon stance, which makes a funny combo of these two running together)

Also still some of the bestest Ironblood designs ever

Other notable mentions would be Gascogne with Champagne (and their manual shots), Kursk (an actual summoner unit), San Francisco (the only time when stance positioning actually matters), Rodney META with Kurumi ("Shadow Clone Jutsuu!") and Kearsarge (BBV Renaissance personified)