What's the maximum bank angle of a glider?
The bank angle ($\phi$) in a stationary turn is limited by the maximal load factor of the aircraft (or the pilot- whichever is less):
$$\phi_{max}=\arccos(\frac{1}{n_{max}})$$

Utility category gliders ($n_{max}=5.3$) have a maximum bank angle of 79°. Aerobatic gliders with $n_{max}$ between 7 and 10 can fly only slightly steeper turns (82°-84°).

If you're flying with an airspeed slower than design manoeuvring speed $V_A$ the limiting factor is the maximum lift coefficient. In that case, the formula gets
$$\phi_{max}=\arccos(\left(\frac{V_S}{V}\right) ^2)$$
$V_S$ is the stalling speed in the current configuration, $V$ the airspeed of the glider.


(own work)

Then, how do you get the tightest turn radius? With high speed and high bank angle or with low speed and low bank angle? Without the structural limitation the radius could become 0. With it, however, your best option to get a minimal turn radius is to fly with maneuvring speed and maximum load factor.
You can use the following diagram to calculate the minimum turn radius at any speed (just multiply the ordinate with the squared stall speed of your aircraft).


As stated in many comments, during aerobatics/dynamic manuevers you can get any bank angle for a certain (sometimes really short) period of time:


0°/180° bank angle Source


90° bank angle Source