southee is 5th at worst, and he, steyn, harris, johnson and anderson are a level above the rest. southee has taken on the anderson/harris like quality of making the attack around him bowl better somehow. those guys are the glue that keep other really good and destructive bowlers like broad, boult, whichever australian is fit this week etc performing. for whatever reason the latter fall to pieces without their leaders.
where you rank him depends on whether you want to go for career vs right now. steyn is the best no matter which way you look at it, but career hurts southee and harris but right now gives them a boost.
i don't know where to put johnson on career, but right now he's godlike in fast and bouncy conditions but needs his babysitter harris to help him out when he leaves the macho lands. i suspect howsie is giving southee a boost to second because southee has ripped india, sri lanka and the west indies to pieces in slow low turning conditions and johnson was nullified a bit, but then johnson had to put up with ****s being useless at the other end. like southee, harris has been crucial to wins in places like sri lanka.
southee has taken good poles at home, in england, in australia (back when he was **** even), the subcontinent and the west indies. south africa and the UAE are the final frontier for him and he hasn't bowled there. he's no longer just a fast medium swing bowler - he's got the bounce, the accuracy and the variations. the only thing he lacks is express pace, but at 135-140 he's sharp enough given his height. this is a brave call from howsie given the depth and competitiveness of pace bowlers around the world but it isn't crazy.
edit: actually me saying johnson needed babysitting is unfair. he bowled very well. he could have used some help though...