My favourite, as many will probably know, is my silver J-8.
I love it for it's smooth bokeh, the softish image and wonderful rendering of the colours. The price was also pretty darn good.
Other favourites of mine are my CV 25/4 and my J-9.
The 25mm is, together with the J-8, my most used lens. I take it everywhere. At f4 it's not really fast but the wide angle gives me lots of latitude; I can safely use 1/8 sec, even 1/4 sec. I use it for street photography (from the hip) and for indoors shots, especially in confined places like Mongolian nomads' tents (I'm gonna use the word ger from now on, which is the proper Mongolian word for them and much easier for me to use in postings 😛 )
The J-9 is similar to the J-8, except that it is an 85mm lens. It also has smooth bokeh, a slightly softish image and renders the colours very nicely. I don't use it as often as I should but it's hard using an 85mm when out in the streets.
I should shoot more with my J-12 on my M2 to get a good feel for it. I know it's very sharp but my experience is otherwise rather limited. I need to see more results to be able to make a founded opinion.
I got a J-3 and a J-11 recently but I haven't developed or scanned any neg of them yet, so I can't say much about the results.
All of these lenses are cheap or, at least, not expensive. That is a major advantage of them and let's me use them in a happy-go-careless fashion. My cameras and lenses get "thrown" into a courier bag or backpack, and have to suffer abusive climate quite often. They've all seen extreme cold, extreme humidity and high temperatures (from the wintery cold of Mongolia to the jungle-type climate inside the gers to the parching heat and dryness of Mongolian summer to the rain and sleet storms in Holland). None of this weather gave me reason to keep a lens inside the house, figuring that if it breaks down it can be replaced cheaply. 🙂