The Hermite-Korkine-Zolotarev reduction plays a central role in strong lattice reduction algorithms. By building upon a technique introduced by Ajtai, we show the existence of Hermite-Korkine-Zolotarev reduced bases that are arguably least reduced. We prove that for such bases, Kannan's algorithm solving the shortest lattice vector problem requires~$d^{\frac{d}{2\e}(1+o(1))}$ bit operations in dimension~$d$. This matches the best complexity upper bound known for this algorithm. These bases also provide lower bounds on Schnorr's constants~$\alpha_d$ and~$\beta_d$ that are essentially equal to the best upper bounds. Finally, we also show the existence of particularly bad bases for Schnorr's hierarchy of reductions.