Oh, the 1* are very much possible... but mostly among older designs or those that are intentionally "retro" not only in appearance / function but also in performance. 
Canon for example had a boatload of lousy film kit zooms a decade ago (or a little more) - it's a good thing you haven't started to test those, or you probably would have been dispensing 1* or ½* ratings left and right. Though I've been suggesting for years that you do one such review for April 1st... 
It's by far not as easy to have a 5* lens, of course. I've been an avid reader for many years now but right off the bat I can remember only one 5* review. Unsurprisingly it's of the Canon 70-200/2.8 IS II (on APS-C; I thought it was more like 4.5* on FF). Have there been more? It looks like something always gets on the way, and it's darn near impossible on FF because the vignetting is always higher there and that inevitably shears off anywhere between ½* and 1*... Most wideangles are also out because they have barrel distortion as a rule... Ditto all the ultrafast lenses because of the CA...
EDIT: Aha! I've found one more 5* review but I knew where to look - it's one of those rare lenses to own a system for, and reviewed on APS-C to boot. Everyone here probably knows what lens I'm talking about. I was a bit surprised that the Sony 135/1.8 didn't make it but as I've said... the CA present did it in.
Canon for example had a boatload of lousy film kit zooms a decade ago (or a little more) - it's a good thing you haven't started to test those, or you probably would have been dispensing 1* or ½* ratings left and right. Though I've been suggesting for years that you do one such review for April 1st... It's by far not as easy to have a 5* lens, of course. I've been an avid reader for many years now but right off the bat I can remember only one 5* review. Unsurprisingly it's of the Canon 70-200/2.8 IS II (on APS-C; I thought it was more like 4.5* on FF). Have there been more? It looks like something always gets on the way, and it's darn near impossible on FF because the vignetting is always higher there and that inevitably shears off anywhere between ½* and 1*... Most wideangles are also out because they have barrel distortion as a rule... Ditto all the ultrafast lenses because of the CA...
EDIT: Aha! I've found one more 5* review but I knew where to look - it's one of those rare lenses to own a system for, and reviewed on APS-C to boot. Everyone here probably knows what lens I'm talking about.
I was a bit surprised that the Sony 135/1.8 didn't make it but as I've said... the CA present did it in.