The correct order of basic strength in water for guanidine, methylamine, pyridine and aniline is

The guanidinium ion is stabilised by resonance among three equivalent nitrogens (pKa about 13.6), making guanidine the strongest base. Methylamine (conjugate acid pKa about 10.6) is a typical aliphatic amine. Pyridine (pKa about 5.2) has its lone pair in an sp2 orbital and is more basic than aniline (about 4.6), whose lone pair is delocalised into the ring.