Hey Carlos,
if you mean the biggest by size, on the Onefinity CNC you can mount a 80 mm diameter spindle with the optional 80 mm spindle mount. If you mean the biggest power, this would mean 2.2 kW, as more powerful spindles usually have a larger diameter. If you mean biggest milling bits, you should buy a spindle with ER20 collet size since this will allow you to use milling bits with shank diameter of up to 13 mm (including 1/2″ shanks).
If you mean a big sum of money, I would recommend Mechatron ATC-8022-42-HSK25 2.2 kW 42,000 rpm water-cooled (unboxing video) with HSK-E 25 machine taper and automatic tool change (automatic tool change video), a cooling station and a ready-to-use electrical and pneumatic control cabinet with Omron MX2 2.2 kW VFD inside.
If you want something simpler but still industry-grade, you could go with HFS-8022-24-ER20 2.2 kW 24,000 rpm water-cooled (datasheet), buy an empty control cabinet and equip it yourself with a VFD and the rest. At least two persons in this forum own this spindle. Note that it is recommendable to always buy the spindle cable from spindle manufacturer.
Note that some people replace their z axis assembly (see also here and here) which would allow for even larger spindles.
Welcome to the forum!