I've had some excellent results on my Canons using all sorts of lenses. I currently use Contax Carl Zeiss lenses (exceptional quality, my preferred lenses) but these are expensive. A good starting point are the M42 Takumars (55/2 and 55/1.8 are often cheap or the 50/1.4 is a super lens if you can get one for £50.00 or so).
Carl Zeiss Jena lenses provide lovely IQ but are a real pain to find a good one. They suffer from mechanical problems so make sure you buy from a reputable dealer if you take that route. The 50/1.8 Pancolar, 35/2.4 and 35/2.8 close focus Flektogon are great lenses. The 135/3.5 Sonnar is easier to find and is incredible! Grab one for £50.00 and you'll be laughing
Cheaper options to the Sonnar are the Jupiter 11A and Jupiter 37A which are clones of the Sonnar.
The Pentacon 135/2.8 preset has the smoothest bokeh I've ever seen, so is another one to watch out for.
All the old Nikon lenses are great value too as are the Olympus Zuikos and Yashica ML lenses. A really cheap starter lens is the Helios 44/44-2/44M/44M-4 which is a 58mm f2 lens based on the Carl Zeiss Biotar. It has interesting swirly bokeh
When using manual lenses you will need to use either manual mode or aperture priority mode (my preference) as the camera can't adjust the aperture (you do that on the lens) but it can control the shutter speed
It's a bit fiddly at first, but well worth the effort IMO. I tested the kit Canon 18-55mm lens up against an Olympus Zuiko just recently and the Olympus was miles better, even wide open at f2!
As you can see, I'm a bit of an MF fan
