Basically, anything that was in Tekkit Classic (back in Minecraft 1.2.5). If I had to pick just one, EE2 or as it's known now Project E, that mod makes life so much easier. Project Red (or Red Power back in the day) is also a nice one, allows Redstone to climb vertically and run along ceilings and also adds a lot of logic gates and stuff to make advanced Redstone systems more simplistic. I like a lot of the new mods but honestly Tekkit Classic had the perfect collection of mods, these days I feel everyone just bundles too many together and they don't have any synergy between them. I haven't played Minecraft in a long time so Project E and Project Red might not even be around anymore but hopefully, they are.
Well, I was never on the old MBs or Minecraft Galleries but my favorite mod here is... I feel like I should NOT say 'cause all the mods are great and helpful and awesome. TheGreatCon is the best mod