Favorite Mod is a big question since my preferred way to play modded Minecraft is within a modpack with lots of mods working together to create a deeper MC experience. My favorite modpacks have always been FTB packs, ever since FTB Ultimate all the way back in 1.4. I loved the great balance of magic and tech mods their packs always had. My favorite mods have changed over the years, but if I had to create my dream modpack in just 10 mods, it would probably be something like this:
10: Optifine - low rank because despite its practicality, it still causes major issues with a lot of mods and can be a general pain in the ass sometimes. used best in vanilla+
9: Tinker's Construct - very fun and useful mod, great way to make late game OP tools, plus me like big lava pit :)
8: StarMiner - old ass mod from back in the day that used to be super popular. you could use it to affect gravity and it was mind blowing shit at the time and still is despite its age
7: anything by iChun - iChun is one of the most prolific Minecraft modders with literally everything he touches becoming a banger mod how does he do it
6: MrCrayfish Furniture - hands down the best furniture mod you can get. all the models in this mod have been worked perfectly to fit in almost any build ever and a good bunch of them even have useful functionalities
5: ComputerCraft - the main thing I like about CC is that it's essentially an entire command line computer framework running natively inside Minecraft all while being really easy to understand and surprisingly powerful for what it is
4: ProjectE - if you like a balanced modpack, do not install this mod, because this mod makes creating expensive things super cheap and super easy. almost every item with a crafting recipe that needs vanilla blocks gets assigned an EMC value, which can be used transmute new things, i.e. trading 64 dirt at 1 EMC per block, for something with an EMC value of 64.
3: Twilight Forest - probably the best modded dimension mod you can get, it's like Aether on steroids, and also a sadomasochist. great terrain generation and awesome bosses. if you haven't played it, play it now.
2: Mekanism - finally, a better energy and automation mod than IC2!!! this mod is a blast to learn about and use, with power generation solutions for everything early to late game, as well as helpful things like multitools, jetpacks, and Long Fall Boots from Portal.
1: Applied Energistics 2 - literally the mod ever. I have not found a single mod that feels as good to progress in as this mod. I love seeing huge ME system setups on the FTB subreddit so fucking much it's so good. ITS LITERALLY INFINITE MODULAR STORAGE WHAT ELSE DO YOU WANT
Honorable Mention: Create - Being somewhat of a Minecraft veteran, I've seen several mods try to do what Create does with varying degrees of success. UgoCraft was a very old mod that allowed blocks to rotate like cogs in Create, resulting in awesome builds and kinetic creations, however it had very poor performance, crashes a lot, was very unpolished, and never got updated to recent versions. Archimedes Ships was an old mod that worked great for making block entity ships, but the mod was overall just another transportation mod. It genuinely blows my mind what people can do in Create, as I never though you could make anything like that in Minecraft, even with mods. Anything in the game can move any way you want it to, which is awesome! It's It's like RotaryCraft but not omega cringe! Unfortunately, I'm not well versed in how the mod works, so I can't say how much I like it.