I don't think Jellicent should be ranked. I mean, sure, it cockblocks Keldeo, has Will-O-Wisp, and reliable recovery. I mean, it has a minuscule niche of fitting onto stall/defensive teams, but then you realize that stall really doesn't exist. While I'm not saying it's bad or anything, it's usefulness and usage has really declined. Bulky offense (or balance) is really the way to go, and you're recommending a Pokemon for a playstyle that really doesn't exist to be honest. Secondly, you mentioned yourself one of Jellicent's most crippling weaknesses - Knock Off and Pursuit. With Pokemon such as Tyranitar, Bisharp, Weavile, Scizor, Azumarill, Mega Gyarados, and to an extent Landorus and Clefable, roaming free, this weakness is amplified much more, making Jellicent more often a liability than not. Thirdly, Jellicent has a terrible matchup against... well... pretty much over half of the tier. Looking at S to A+ to A ranks, Jellicent really only checks Keldeo, Heatran, and Hippowdon. If you're lucky, maybe Latias and Gliscor as well. There's really not much Jellicent can actually do in this metagame, especially when it is beaten by so many Pokemon, and as I said earlier, it's more often a liability than not. Lastly, Jellicent mandates a lot of team support. Because of its substantial amount of weaknesses, there must be a lot of team support required to help Jellicent function - just thinking at the top of my head, it needs Dark-type, Pursuit, and Knock Off absorbers (at least 2), such as Clefable, Keldeo, and Tyranitar, checks to various Pokemon such as Mega Metagross, Landorus, Mega Altaria, Mega Manectric, Raikou, Rotom-W et cetera that completely destroy it.
TL;DR Keep Jellicent unranked.