There are so many - it really depends on what you want to go for... So I'll just mention some of my personal favorites.There are a number of different lenses from Meyer Optic Görlitz, which are known for their unique rendering:
Meyer Domiron 50 mm f/2
It has a very unique (often somewhat busy, but interesting) rendering and excellent 3D pop
Don't be another brick in the wall. Stand out! by simple.joy, on Flickr
Backyard rocket launch by simple.joy, on Flickr
Bubble industree by simple.joy, on Flickr
Then there's the Primoplan 58 mm f/1.9
Extremely smooth and pleasant rendering with a round aperture
Light … outside and inside by simple.joy, on Flickr
Small budget Christmas tree by simple.joy, on Flickr
Gotta hand it to the rain: it feels amazing! by simple.joy, on Flickr
If you're not opposed to adapting lenses via a helicoid + an adapter/clamp there are also a couple of fantastic projection lenses out there:
Meyer Kinon Superior 5 cm f/1.8
Fantastic for images with focus on the center.
Nature needs no open letters… by simple.joy, on Flickr
Glowing pre-views by simple.joy, on Flickr
Isco-Optic Ultra Star HD Plus 75 mm f/2.1
Ultra-smooth.
Cookies - and all worries are melting away... by simple.joy, on Flickr
and finally some of the longer focal length projection lenses offer a lot of flexibility for bokeh experiments, like that:
Braun Ultralit PL 85-150 mm f/4
Trying to bokeh hole in my idea? by simple.joy, on Flickr
If you're into close-up/macro photography some enlarging lenses offer very interesting possibilities as well. You may or may not like distinct bokeh shapes, but I personally love experimenting with that and there are some great ones among enlarging lenses:
Steinheil V-Cassarit 50 mm f/3.5
Rust suspect busted! by simple.joy, on Flickr
You‘ve finally match your master! by simple.joy, on Flickr
Meopta Belar 50/75 mm f/4.5
Usually that's how well I fit in... by simple.joy, on Flickr
I‘m completely crazy… but a square shooter! by simple.joy, on Flickr