With disregard to camera brand or model, the zoomz which will give you more bokeh would be from the f2.8 group in the 70/80-200 mm range, and up to f4 in the 300-400mm range.
The 'secret' to creamy soft and smooth bokeh would be a larger number of apeture blades that form a more concentric and 'perfect' circle. For example, 5 blades form a pentagon, while the professional or luxury series of lenses tend to feature 8 or more blades to get closer to that 'perfect' circle. If you're familiar with the mechanical construction of multi-blade apetures, you'll realise that it's a trade off as incorporating too many blades would complicate construction and increase cost. There may also be issues with the consistency of the blades closing down if too many blades are used.