In general terms, which category of impression material is considered most dimensionally stable for clinical use?

Regular-set elastomeric impression materials generally offer the best balance of adequate working/setting time and dimensional accuracy. Very-fast-set materials can be more prone to handling errors and exotherm-related distortion, while overly slow-set materials risk distortion from movement/delay before pouring. (Material class — polyether and addition silicone vs. condensation silicone or alginate — is usually the bigger determinant of stability than set speed alone, but among set-speed options, 'regular' is the conventional answer.)