What's the current strongest Chess Engine built solely on heuristics, no neural networks or any kind of deep learning?
The strongest classical chess engine is probably Stockfish classical, i.e. Stockfish with its NNUE turned off. I say this because Stockfish was leagues ahead of all its CPU rivals before the NNUE revolution, and after the NNUE revolution very few people are still working on classical engines, so it's unlikely anyone caught Stockfish. In fact, if you look at CCRL (Computer Chess Rating Lists), no classical engine has caught Houdini (all the engines above it are either NN or NNUE based).