If you're ruling out the big bang, then the most extreme releases of energy in our universe should be cases of runaway gravitational collapse. There is a rigorous theorem in general relativity (Penrose singularity theorem) showing that these will generically lead to the creation of singularities. For realistic gravitational collapse, it's expected that in the end state of this process you will have a black hole, which has an event horizon surrounding a certain specific type of singularity described as spacelike and not a strong curvature singularity (not s.c.s.).
However, during the initial process of formation of the black hole, it's not really fully established what kind of singularity you would have. It could be timelike rather than spacelike, could be an s.c.s., and might even not be surrounded by an event horizon (which would violate the cosmic censorship hypothesis -- but we don't know whether the CCH is true or even the best way to state it). If it's an s.c.s., then general relativity predicts that the infalling matter will be infinitely compressed, and therefore probably heated to infinite temperature. GR is a classical theory, so this should probably be interpreted as a statement that an s.c.s. would heat the matter to the Planck temperature.
So if an observer were to jump into a black hole during its initial formation, and if the observer was able to withstand the temperatures, then they might get a millisecond during which they could observe the matter around them being heated up to very high temperatures. Whether these temperatures would rise to the Planck temperature is not really known (probably not), and whether any of this might ever be observable from far away, without suicide, is not really known (but probably not).
Straight from my 7 year old to you, exactly what it says on the cover: What is the hottest thing in the universe?
So at this level, scientists don't really know for sure, but they think if you jump into a black hole while it's in the process of being born, you might be able to see matter heated to extremely high temperatures, probably hotter than anything else in the universe since the big bang.