Just one indeed! Choosing my favourite vintage lens depends on my mood, the weather, what I'm trying to achieve - e.g. close ups, or big bokeh, or portraits, or landscapes, plus sentimental value (and my first ever film lens, the S-M-C Takumar 55/1.8 wins on that account).
However, if someone said: 'you have an hour to take a photo with only one lens; a photo that other people will find interesting', then I'd always go for the Macro-Takumar 50/4 pre-set. It does everything really well from 1:1 macros to far away landscapes, and excels on modern digital sensors. It's also a joy to use, like a great scientific instrument.
On the other hand, if I just need to have fun and re-ignite my enjoyment of photography and it's mysteries (never sure exactly how the photos are going to come out), then any of these Helios 44s would be perfect. The silver M39 Helios 44 at the front is one of the best lenses, optically, I own. Right up there with the best Takumars, Zeisses and others. Maybe it's an exceptional copy, but it is exceptional, even better than its 'father', the Carl Zeiss Jena Biotar 58/2.
Mad, bad and dangerous to know - along with the radio-active S-M-C Takumar 50/1.4, this even more radio-active Carl Zeiss Jena Pancolar 50/1.8 is another favourite vintage lens. I just love the rendering it delivers.
And finally, the lens I lusted after for so long - a very early version Helios 40. I thought it would instantly become my favourite lens. It takes unique photos but it's too big and too heavy to enjoy all the time...too much like the most recent FF digital behemoths perhaps! (PS Shots from all my lenses arranged into albums are here:
Simon's utak?s albums | Flickr).