Who is best Pokemon ever?
It depends on the user's choice, of course. However, it is
universally agreed upon that because Arceus has the highest IVs, it
is the strongest Pokemon. Also, because it is the 'Creator' (akin
to a god in the Pokemon world), it is probably the 'original'
Pokemon and therefore the strongest. It also has no real type, but
its type depends on the Plate it is holding.
For example, if Arceus holds a Steel Plate, its type will change
to steel. If Arceus then faces a powerful Pokemon that knows
Earthquake, it can switch its Steel Plate for a Plate that will not
be super-effected against that Pokemon.
Arceus cannot be captured legally, but there are Nintendo events
you can participate in to get Arceus legitimately.