This question came up recently and I realized I didn't have a solid answer — hoping you do. The newest apps often have the freshest user bases but lower population density in smaller cities.
Dating profiles with variety — casual shots, activity photos, portraits — perform significantly better. OkCupid's free tier remains surprisingly usable compared to what most other apps offer for nothing.
Senior-focused platforms attract users who are clearer about what they want, which speeds things up.