Favourite lens? Ooh, I've had so many over the years, it's very hard to answer. Right now, with mirrorless cameras meaning I can use all those lovely vintage lenses again, I'm in lens heaven. I've got various adapters for my Lumix m4/3 cameras, and for my full-frame Sony A7, and I'm currently loving...
Vintage...
* Voigtlander Color Ultron 50/1.8. A 7-element Planar design, spectacular on the Sony. Possibly my favourite ever 50.
* Helios 58/2, the Russian "swirly bokeh" lens. Not for everyday use, but lovely when it's suited to the subject matter (I have two versions, mk2 and mk4, slightly different). Really only works full-frame, so on the Sony
* Olympus Zuiko 135/3.5. I use this on m4/3 with a 2x crop factor, so equivalent to 270mm. And the great Lumix body stabilization makes it easy to hand hold.
* Mir 35/2. Another Russian lens that's way better than its cheap price suggests, works well on full frame. (And speaking of Russian 35s, I was disappointed to find my Jupiter-12 is no good on my Sony - with a digital sensor, it completely loses definition away from the centre)
More modern lenses...
* Canon 50/1.4 and 85/1.8 (on a 5D2). Really nice implementations of two of my favourite focal lengths, and not bulky like their faster siblings with the red lines.
* The fixed 23/2 (35mm equivalent) on the Fuji X100F. A great walkaround focal length, and coupled with the Fuji sensor, the colour rendition is beautiful.
Sold but wish I hadn't...
* Olympus Zuiko 50/1.4, sold before I got into mirrorless cameras, must get a replacement.
* Voigtlander (modern Japanese) 35/1.4 Nokton. Favourite lens on film rangefinders for years, again sold pre-mirrorless. Would love another one, but they're not cheap.
* Leitz Summitar 50/2. It almost physically hurts when I remember I had one of these but sold it. And since then the price has increased quite a lot.