Lileep is far from S-Rank. Being weak to Fighting-type moves is its biggest problem. Lileep is useless against nearly all Fighting-types in LC. No competent player is switching their Mienfoo into a Lileep in the early unless you're 100% sure it's going for a move that's not Toxic. During the lategame where Lileep has born worn down by weak hits and some residual damage, then that's the time to switch Mienfoo into Lileep so u can get off a Hi Jump Kick or grab momentum with U-Turn. Lileep sucks versus Croagunk, Scraggy, and Timburr, all of whom are on nearly every Little Cup team. Lileep is an excellent check to a variety of Pokemon like Murkrow and Water-types but it has a pretty significant weakness to Fighting-types.
Finally, Lileep has absolutely no offensive presence. Pokemon like Ferroseed and Bronzor are free to set up hazards in front of Lileep with little fear. Lileep is also DESTROYED by Toxic. A Toxic'd Lileep is a bad Lileep.
Pokemon in this tier have very few flaws that are patched up by numerous positive traits.
The flaws I've listed out are significant enough to keep Lileep out of S-Rank. Personally, I think Lileep may even be a B-Rank Pokemon. It's very good at what it does but its combination of 4ms, weakness to Toxic, and weakness to Fighting-types are a little too important for A-Rank.