[stoppingdown]

My first answer would have been the Helios 58mm f/2, but you already got it. Another legacy lens that I own is the Trioplan, but it's a much more controversial lens - we discussed about it in the past, and some love it, some hate it. Furthermore, it's among those that are expensive, which makes things more complicated. A third lens that I really enjoy is a Nikon 50mm f/1.8 AF-D.

Most times I use all these three lenses with adapters embedding focusing helicoids, for macro (flowers and critters). The three lenses have very different characteristics: the Nikon, in particular, is the one with a "normal" behaviour (smooth bokeh, but without exotic effects) and it's the sharpest (even wide open: for instance, the Helios, wide open, is sharp only in the centre).

Sorry for the self-citations, but some images are needed to clarify my points. Here you find a comparison of the three lenses for macro:

http://stoppingdown.net/blog/three-different-bokeh/

Here some more info for what concerns aberrations:

http://stoppingdown.net/blog/bokeh-and-other-optical-qualities-with-extension-tubes/

These are by far the shots that gave me the best results with the Nikon:

http://stoppingdown.net/blog/zygaena-filipendulae/

Note that the Nikon should be relatively inexpensive, in any case I have an auto-focus version. Of course auto-focus in this context is useless, but it's a lens I inherited from my previous Nikon life. Should I buy it from scratch, I'd evaluate a cheaper, manual focus version. But even though 50mm are usually easy to produce and have good performance, I'm not sure the manual, older versions have the same quality (it could be worse, but it could be also better, I really don't know).

Note that manual focusing with macro (and flowers, which easily move, not to talk about critters) requires loooots of patience and tons of shots, to pick keepers from. I sometimes wonder to have myself the patience for that job. But it's rewarding.

Of course I've talked about macro, but legacy lenses might be good also for other stuff, such as portraiture, but it's not a field I'm interested to.

Phillip Reeve (https://phillipreeve.net/blog/) publishes a rich and ample review of manual lenses to be used with adapters, you could start and have a look there.