octopuses is the most common form in the UK as well as the US;
octopodes is rare,
and octopi is often objectionable.
The Oxford English Dictionary lists octopuses, octopi and octopodes (in that order);
it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun, which it is not. Rather, it is (Latinized) Ancient Greek, from oktṓpous (ὀκτώπους), gender masculine, whose plural is oktṓpodes (ὀκτώποδες).
If the word were native to Latin, it would be octōpēs ('eight-foot') and the plural octōpedes, analogous to centipedes and mīllipedes, as the plural form of pēs ('foot') is pedes.
In modern Greek, it is called khtapódi (χταπόδι), gender neuter, with plural form khtapódia (χταπόδια).
Use "-i" for plurals when the word is borrowed from a Latin word that used "-i" for plurals. Note that in Latin not all words that end in "-us" are made plural by changing the "us" to "i". For example, in Latin the plural of "locus" (place) is "loci", but the plural of "fructus" (fruit) is "fructus".
Octopus comes from Greek and not Latin, and so does not follow the Latin rules.
Best Answer
I would go with octopuses.
That is part of the Wikipedia "Plural form of words ending in -us" article: