Mediocre means average but it has a negative connotation. Mediocre at best means if you're being nice, you'd say it's mediocre... but it implies you really think it's a bit rubbish. I don't think I've heard anyone say rankle.... maybe it's old-fashioned or more American English? Either way, the internet says it means irritating

Swop the words a bit: at best, he’s mediocre. Meaning he is poor or bad, and the best he can hope for is to be average. Rankle is a word that means something caused annoyance or resentment probably for some time: the way someone was treated, or a comment for example.