The official recommendation is as given above in the earlier replies.
Having shot wildboar both in UK and Europe, although I am sure there will be more than a few on here who have shot more, I would offer the following advice:- in many European countries the minimum recommended is 7mm although again in relatively few countries is there a legal minimum. That is common sense. As
@Jelen points out a minimum ME may be more sensible. Being purely pragmatic I would ask how comfortable you are with perceived recoil? If you are tolerant then by all means "go big". If less so, or because you want to use a very lightweight rifle (like some of the break action single shot rifles), then perceived recoil becomes an issue. In which case I would personally recommend something more like 7x57 or 7mm-08 or similar. If you want a magnum calibre then any of the 7mm magnums are good (the Rem mag probably enjoys the least perceived recoil in that calibre than the other 7mm Magnums) or a 7x64 which has almost identical ME.
The two largest boar I ever shot were in UK, escapees from a farm and were shot with a 7x57. Both were comfortably over 200kg eviscerated. I would recommend bullets weighing more than 160 grains as in medium calibre rifles they are more likely to have the construction and sectional density to achieve desirable penetration; 175-200 grains is generally better.
Like
@Foxyboy43's video I have shot very large feral pigs in the Australian bush with a .22lr held in one hand whilst riding a motorbike whilst alongside them and shooting directly downwards between the shoulder blades. Penetration is an issue though! Humanity dictates we should aim for a quick death (or at least reduction of consciousness) and larger calibres generally achieve this.
Personally I now use a 7mm Rem Mag because with a 175gr bullet it works effectively. I have used .22lr, 6.5x55 and 6.5x57, 7x57 and 7mm Rem Mag - all killed but some quicker than others.