If it's screwmount, you can get two great lenses if you don't need speed—the Voigtlander 35/2.5 and the 50/2.5. They are both very small, handy, sharp, and with excellent contrast. The 50 has a great vintage look to it, but with excellent contrast. The built-in hoods with push-on caps make them super handy to use. Add LTM to M adapters if using on M-mount.
If its just a 50, the Konica Hexanon 50/2 has a smooth, clean look, and very sharp. The built in hood with push-on cap makes it a pleasure to use. The lens feels very solid and well made. About $400.
The Summicron 50 is a great lens, very sharp. To me it creates a harder edged image, more etched, than the Hexanon. About $500.
For a less-well built lens with a great Sonnar look, buy a Jupiter 3 if you can have it collimated by Brian Sweeney to focus properly at close distance at f1.5. Softer contrast than the above lenses, very sharp, with beautiful out-of-focus areas. Total cost perhaps around $100, plus LTM to M adapter if needed.
I'm sure there are other good ones.
Oh, that soft contrast in the Russian lenses. So sweet.
I think for me the 35-50mm focal length is where I would get the most use. 28 is a bit wide for the "one" lens starter kit. I have been reading about the CV 40mm's as well as their 35's and 50's. This lens would be going on an M2 body.
Of the three, the Voigtländer spends most of the time on the camera. I do a certain amount of low-light in smoky metal bars, and the f/1.4 is the correct tool for the job here. The 1.2 would have been nice in terms of the additional n'th of a stop of aperture, but having read a number of reviews comparing sharpness and overall performance, I went for the 1.4.
The Summicron is my 'kit' lens which is being a trifle unfair, as it's possibly the sharpest 50mm I've ever seen, feels like a jewel in the hand, bla bla gush gush. I wish it were just a tad faster. I may one day see if I can get hold of something in a 1.4 as well.
The (1950's era) Elmar was got for a good price (well under €200), is in good condition, and I needed a 'portrait' lens. Yes, there are better M-mount lenses in this range, but I wasn't going to argue that price for a piece of vintage Leitz iron. Again: when I am big and clever, I may obtain a more 'precise' short tele in the 70 to 90 range. But for now the Elmar will do.
Any of these three, depending on what you want to use the camera for, would be a good 'starter' lens.
Every one is different but, I'd suggest a 50mm as a first lens, then perhaps a 35. The M2 has no frame lines for the 28mm. Remember that the M2 has only frame lines for the 35, 50 and 90mm lenses. (My 90 and 135 are seldom used.)
The Voigtlander 15mm Heliar would certainly be a interesting choice as well. I have this lens and it certainly opens up some unusual opportunities.