I'm no mathematician, but I think the joker switches the rankings of full houses and flushes.
With 53 cards, you have c(53,5) or 2,869,685 possible poker hands. I'm using this site http://en.wikipedia.org/wiki/List_of_poker_hands as a reference for normal deck hand numbers.
5 of a kind
This has to be exactly 4 cards of the same rank plus the joker. So that means there are exactly 13 five of a kind hands. 13/2,869,685 = % 0.000502
Straight flush
You would have all of the normal sized deck straight flushes, 40, plus all of those with one card replaced by the joker, 40 * c(5,1) = 200. 240/2,869,685 = % 0.000923
4 of a kind
For this one you use all the regular deck four of a kinds, 624, plus all the joker four of a kinds. One card is the joker then three of a kind, 13 * c(4,3), then the 5th card can't match the three of a kind so there are 53 - 5 = 48 cards left. 624 + 13*c(4,3)*48 = 3120/2,869,685 = % 0.12
Full house
Normal deck full houses, 3744, plus jokered full houses, c(4,2)*c(4,2)*13*12. 3744 + c(4,2)*c(4,2)*13*12 = 9360/2,869,685 = % 0.36
Flush
Normal deck 5 to a flush, c(13,5)*4, plus four to a flush c(13,4)*4, minus all straight flushes. c(13,5)*4 + c(13,4)*4 - 240 = 7768/2,869,685 = % 0.29
Straight
A normal sized deck has 10,200 straights so that will still be more common than flushes and full houses.