Max percentage of attack speed?

Bonus attack speed in % is not capped. You can get as high as you want. What is capped is the actual Total Attack Speed, displayed in Attacks/Second, and it is capped at 2.5, except for Kog'maw when he uses his ability when it is capped at 5.

It depends on the champion. Each champion has a different base attack speed and a different percentage growth per level, so they'll all need different percentages to reach the maximum possible attack speed, which is 2.5 attacks per second. For example, Annie would need 308.12% to cap her attack speed, while Kennen would only need 203.77%, and this is because of their base values.

Then...I think / at 50% is highest. Hurricane used to be at 70% but they shifted some of it's stats to crit% is also 50% if you count stuff from other maps than SR as viable information.

Edit: also has 120% on her E for 3 seconds at max rank, if you're counting champ skills/buffs.

So...I guess eve with 6 and her E would give her 420% AS....then you could add in runes (41% with full AS page) and 4% from masteries, for 465%, then tack on some AS buffs like from an ally with a shield/heal, blood boil... essence flux, hunters call, etc.... But all of this gets pretty pointless because while you may be gaining %AS, you're still gonna be capped at 2.5 attacks a second, unless you're , then you're capped at 5..which he can do on his own without ally buffs mostly.

Unless this is all for passive...then half of what I said was moot.