My big 4 for any modpack I put together (disclaimer, it's been a year or so since I was last big into modded MC, so my knowledge may be out of date and it's possible these don't all work together on the current version):

- Buildcraft. It's often considered old and dated, because it gives only a few simpler building blocks which need to be combined to make anything useful, but I consider that it's strength. It basically provides a piping system, for taking items, liquids out of and into inventory and machines, a power system that runs along similar pipes, and machines that will mine or pump for you when provided with that power. Its final stage power production consists of pumping oil, refining it and burning it, though it's underpowered compared to pretty much every other power source as it was one of the first and didn't participate in the mod arms race as much. I like the visual nature of its piping system (you see individual items travelling along the pipes) compared to many of the modern more efficient alternatives.

- Mekanism. Been a while since I played, but there a couple of mods that give you your basic set of processing machines that run on electricity. Mekanism has a decent progression system and gives you some room to make processing chains. (Competitors to Mekanism are Thermal Expansion, EnderIO, Immersive Engineering and IndustrialCraft2. Of these, Thermal Expansion and IC2 were in flux last I played Minecraft, often only having nightlies available, EnderIO falls into the "too powerful" issue at times, and IE is the new kid on the block which I haven't tried yet)

- Railcraft. If you get super into Railcraft it's fun building automated rail networks and trying to run as much off rails as possible. Has a very pleasing scope to it, as you can see machines in action.

- Minefactory Reloaded. It suffers from the "magic block" syndrome a bit, but it enables you to automate things that most mods didn't touch, like mob breeding and farming.

Other options:

* Forestry. It has tools for automated farming, though they are a bit "magic multi-block". Also bee-breeding which is a massive rabbit hole of its own and can basically produce anything if you get addons like extra bees or magic bees.

* Thaumcraft. This mod kind of resists automation of its own progression a bit (though it provides plenty of tools to automate other mods), but it gets in there because it's so different to most other mods out there. Magic and automation via little golem minions that run around, lots of cool items and tools.

* Frames. I think the current mod supporting them is Remain in Motion? Basically allows you to move a lot more stuff around in the world. Vanilla has a super lite equivalent in slime blocks these days, but frames go so much further.

* Project Red. If you ask anyone who's been around for a while, there's a mod called Redpower 2 which was available for 1.2.5 and 1.4.7 that was super popular. Project Red is basically an open source clone of this mod. While last I checked ProjectRed doesn't include its Blutricity or Forth based computer, it does include Red Alloy Wire, which is redstone on steroids that you can run up walls or have lines running in parallel that don't connect etc.

* Computercraft. Build computers that can run lua programs. I often use it for powering displays that show me the status of various aspects of my base. Also little automated turtles that can move about and mine or place blocks from their inventory or other things. Definitely OP but it's fun so I let it pass :p I built a super simplistic vi clone and a package manager for it back in 1.4.