McCullum has moved ahead of West Indian Chris Gayle and India's Suresh Raina, and England's Eoin Morgan has fallen from the No 1 spot to ninth.
McCullum showed how devastating he can be in the shortest form of the game with a 55-ball 91 against India in Chennai on Wednesday, which helped New Zealand claim a series victory.
Martin Guptill is ranked fifth behind Australia's David Warner, and Nathan McCullum is New Zealand's top-ranked bowler at No 5.
It will be confidence boost for the Black Caps ahead of their opening fixture at the T20 World Cup against Bangladesh next Friday night.
They have moved to fifth in the team rankings; Australia's brief but embarrassing stint ranked below Ireland lasted less than a week with the Aussies moving into ninth.
McCullum is likely to be rested in New Zealand's two World Cup warmup games against Australia (Saturday) and South Africa (Monday) as coach Mike Hesson looks to give him time to recover from the painful collision with Kyle Mills in Wednesday's international and allows others opportunities to play themselves into form.
Mills had an x-ray on his cheek overnight but Hesson was confident beforehand nothing was broken.
Mills also has a bruised hip and McCullum was suffering stiffness in his knee and right arm.
Both were lucky to avoid more serious injury given the force of the collision that saw Mills quickly put into the recovery position.
Hesson said he needed to balance the need to give players match practice and the desire to gain some momentum but had plenty of options in a format of the game that often suits New Zealand's players.
"It's a nice problem to have," Hesson said of the options. "It's just the one T20 [win] but momentum is pretty important in this game and probably more the confidence you gain from that so we are really keen to continue that in the Australia and South Africa leg and get some guys in form.
"The key once get get to the Bangladesh game is that everyone is fully confident about their own game so we will give a number of different people different opportunities."
New Zealand have been pooled with Bangladesh and Pakistan at the Twenty20 World Cup in Sri Lanka and need to finish in the top two to progress to the super eight phase of the tournament.
- Cameron McMillan of nzherald.co.nz
