This is one of thoe priodical questions that crops up every few years' ISTR answering it before, but having a peek back in time, last time it was a slight variation,
What is your favourite fixed focal distance?
That was a couple of years back, and then, I plumped for the fixed 35mm Zuiko f3.5 in my trusty old Olympus XA2 35mm film 'compact' that went everywhere with me (and often still does!) for thirty odd years, as probably my 'most-used' lens; though recent audit of negatives vs digi-pics, possibly gives lie to that idea.
In more recent times, the Nikon 'Kit' 18-55 is probably the most-used, certainly on the electric-picture-maker.
For my 'favourite' lens, last time round, I offered the Panomar branded Sigma 12mm fish-eye, originally mounted for Olympus OM, when I bought it twenty odd years ago at a camera-fair, but re-mounted to M42 in recent years. Less well used, but a lot of fun, and prompting me to buy the real-deal 'full-round' 180 Degree FoV Sigma 4.5mm Fish for crop sensor electric picture-maker, a couple of years back. Even less 'used' than the 12mm ever was, and certainly not really endeared itself to me anywhere near as much. So, struggling a little on this one now.
I am rather fond of the M42 fit, 50mm Ziess f2.8 for my old 35mm Sigma 35mm film camera, and its rather nice used on adapter on the electric-picture-maker, too, but hard to say exactly 'why'. It's just a nice lens to use I guess.
I think I would have to say, at the moment, my 'favourite' lens, is probably the Sigma 8-16 UWA I bought just after Christmas, for the simple reason that the novelty hasn't worn off yet and I am actively looking for excuses to use it. Having always wanted something wider than a 28mm for my film cameras, it's taken my quarter of a century to get one, and I have to admit to being a little under-whelmed by it, now I have it! So its only 'favourite' in the way a horse that gets most bets is! I'm not sure the enthusiasm for it will last.
Tough call, really; bit like asking me what my favourite spanner is; end of the day they are tools to do a job, so I pick the one that suits best whatever the job is!