By far not the one with the highest density (looking at Mike's and Richard's answers here), but Perry Rhodan issue #16, "Die Geister von Gol", was about a planet of 916g surface gravity, making it a #3 entry at the time of posting. ;-)
Rhodan and his crew navigated the surface with tracked multi-purpose vehicles ("Shifts"), which needed their anti-gravs (usually used to let them fly) running at max output just to keep them mobile (and not crushing the crew).
I am not sure if you could call the beings they encountered "life forms" (and neither were the protagonists), as they seemed to be energy-beings, but they were attracted by the vehicle's shields (reacting to stimulus), and drained their power supply, growing in size in the process (feeding, growth), so let's call them "life".
The highest gravity planet to bear human life in the PR universe would be Oxtorne, a 4.8g world. A shipwrecked crew of emigrants managed, with the help of genetic engineering, to adapt to the harsh environment of their world within four generations.