The following blog entry is quoted from George Lepp's "Tech Tips" section of the November 2014 issue:
Question: With present technology in DSLR sensors and lenses, what's the limitation in sharpness, assuming good technique. Is it the lens, or is it the sensor? Or are other factors more important?
Answer: "Even assuming good technique, the photographer is the most limiting factor and must take necessary measures to eliminate camera movement and achieve precise exposure: use of a tripod, control of internal camera vibrations, avoiding blown-out highlights or pixelated, underexposed shadows, working with the lens' optimum aperture, and employing expanded depth-of-field strategies such as focus stacking.
Sensors in professional-level cameras are so capable these days that they're somewhat of a given. Some of the manufacturers have been a bit slow in introducing lenses that make full use of those high-count full-frame sensors, and that has created a market for very high-end lenses from Zeiss, Leica and Sigma with mounts for Sony, Canon and Nikon bodies. But there's a limitation to how many pixels you can put on a full-frame (24X36mm) sensor, the old film format. And that's the reason why medium-format has had a resurgence in the form of digital cameras with up to 60 megapixels (that is, not only more, but bigger pixels); it's easier to build high-resolution lenses for larger sensors, so this is the combination to have now if you seek ultimate sharpness and have unlimited dollars to spend.
The answer to your specific question regarding the relative importance of lens vs. sensor is, therefore, dependent upon the particular sensor/lens combination you're asking about. Technique, sensor characteristics and the ability of the lens to resolve enough information to take advantage of a sensor with more pixels all play a part. And perceptions of sharpness, are, of course, greatly influenced by a number of post-capture factors, such as the quality of an electronic display, the dpi resolution of a printer and the size of the print, the native resolution of a projector and, not least, the standards of the viewer."