That is a nice question, makes us think about our gear.
Assuming OPs question spirit allows me to still keep a digital system like Fujifilm X-Pro that replaces 35mm for convenience, I'd keep a medium format mechanical camera. From what I currently own:
The Pentax 6x7 that I love and use the most would be out, due to electronics;
Bronica S2a, it is just too heavy to have as single camera;
Hasselblad 503cx, that I consider the Pentax 6x7 backup. Much lighter than Bronica S2a, but still a bit bothersome to carry regularly;
Folders, a Seagull 203 or the Super Ikonta 534 (or 531, but don't own one) are contenders but not as sturdy as I'd like;
Minolta Autocord RG3, looks almost perfect choice, but the focus lever is a risky bet for a lifelong choice;
So.... Rolleicord Vb? Light, bright screen, pretty good build quality, great results.
Actually, the perfect answer would probably be a Rolleiflex 3.5F [Edit: that I’ve in the meantime bought]), since Rolleicord quality is a bit lacking (referring to the mechanical side), comparing for instance with the Autocord (exception for the focus lever) and ergonomically is also not great.
If I'd gone for 35mm, easier choice: Nikon S2 (or SP, but I don't own one).