Consider the following amines in aqueous solution, with the pKa of their conjugate acids in brackets: piperidine (11.1), methylamine (10.6), pyridine (5.2), aniline (4.6). Which is the strongest base?

The larger the pKa of the conjugate acid, the stronger the base. Piperidinium has the highest pKa (11.1), so piperidine is the strongest base. Pyridine and aniline are weak bases because the nitrogen lone pair is in an sp2 orbital or delocalised into the ring.