Everyone will have slightly different ideas here.
If you want click-stop apertures, the Jupiter-8 (or 3) are not for you. And I think the Sonnar rendering is exactly NOT what people mean when they say "modern." That said, many (me included) really like they way they draw. And really, they are fairly simple mechanically and easy to regrease.
I have a Canon 1.4, and I think it's a great lens - still classic but quite a modern approach to correction, double Gauss design, relatively stable character across the aperture range (unlike Sonnars). Click stop aperture in full stops. Has an infinity lock - dunno whether this is a plus or a minus.
I also have a Summitar (can't comment on the Summitar/Summicron division) which is a very sharp lens, but doesn't do distant out-of-focus areas well - can be messy or even swirly at times. Renders points as donuts when fully open, and as hexagonal donuts (if you have a later version like mine) at part aperture. Renders foreground OOF in a pretty manner, though, so a nice complement to a Sonnar which if course does the reverse with foregrounds and backgrounds. Period correct for a 111c, and cases for the pairing are around. Aperture is NOT click stop but does have an aperture ring. And an infinity stop, and collapses.
Canon's F2 option was the 50mm f1.8 - these are apparently pretty decent and are common, but the general rule of older lens designs (say pre 1980-ish) is that you probably want to buy a lens which is a stop faster than you intend to use. I got the 1.4.
The most modern handling lens I have is a Rollei 40mm f2.8 Sonnar, which uses a mount by Voigtlaender/ Cosina. It's nice having the more modern style focusing tab and click stops (1/3 stop), but to be honest I think older rangefinder cameras call for setting your camera up well before the shot, not trying to change settings with the camera to your eye. And if you ARE going to use TTL metering to adjust your aperture then click stops are irrelevant.
Still, lots of great options depending what you want. I would suggest looking at a lot of photos filtered by lens (in the gallery here, and flickr etc) to see what rendering you like.