Tough for me. The obvious would be Thermal Expansion, Build Craft, IC2 are there (I go back to old versions sometimes and I'm more into Tech mods). Or the niche ones like Cogs of the Machine, Fusion Warfare, TSON Craft, LightningCraft and more in the 1.7.10/1.8.9 era.
I've used Divine RPG, Twilight Forest and Aether a few times but don't have that much attachment to them. While I started with 1.4.5 the 1.7.10 to 1.11.2 era is the era I look back on and play still from time to time.
I'd also say maybe the Rift mods (1.13/1.13.2) era as I restarted Youtube then, nothing noteworthy made for it but still sometimes I remember well (says a lot about my mod experiences huh).
More the larger contents ones I use a lot I remember fondly less the smaller or based on a license ones like the Dalek/Weeping Angels mods which are cool of course but never stood out to me like mods that felt like 'Minecraft' originals.