Calibration of radio telescopes usually uses cold sky and hot ground, rather than hot sky. However, for completeness here is hot sky.
(Cold sky is best achieved by pointing at Polaris.)
If by “hot sky” you mean a direction with a high radio brightness temperature, there are several good targets depending on frequency.
At 1420 MHz (the hydrogen line), some of the hottest regions are:
Galactic Centre
Galactic Center
One of the brightest radio regions in the sky.
Contains strong synchrotron emission, ionized gas, and abundant hydrogen.
Brightness temperatures can be hundreds of kelvin above the quiet sky background.
Coordinates approximately:
RA 17h 45m
Dec −29°
From the UK it culminates low in the southern sky.
Milky Way Plane
Pointing anywhere along the bright band of the Milky Way generally gives a hotter sky than pointing away from the Galaxy.
Cassiopeia A
Cassiopeia A
One of the strongest radio sources in the sky.
RA 23h 23m
Dec +58°
A favourite calibration source for northern observers.
Cygnus A
Cygnus A
Extremely powerful radio galaxy.
RA 19h 59m
Dec +40°
Another standard calibration source.
For the Absolute Hottest Signal
If you simply want the highest antenna temperature, point at the ground!
The ground is roughly at ambient temperature:
T \approx 290\,K
whereas a quiet patch of sky at 1420 MHz may be only:
T \approx 5\text{–}15\,K
This large contrast is often used for calibration. A simple “hot/cold” calibration compares:
Hot load: ground or absorber (~290 K)
Cold load: zenith sky (~5–15 K)
For a hydrogen-line telescope like yours, a common “cold sky” reference is near the North Celestial Pole (Dec +90°), away from the Milky Way. Comparing that with a scan through the Galactic Plane provides a clear increase in received temperature due to Galactic hydrogen and synchrotron emission.