The first mod I played was IndustrialCraft2. Adds a couple of extra tiers of technology (Your first steps will be building machines to double ore output and craft an electricity-using mining drill). If you managed to keep a spare copy of 1.2.5 and are of a technical mind (Or you have ever built something in redstone more complicated then an RS Latch), I would highly recommend Redpower 2. Or, perhaps you just want new terrain features to look at, in which I suggest giving ExtraBiomesXL a try.
Right now I'm starting over from scratch with Feed the Beast, and have added Optifine, Matmos, and Smart Moving, and I am loving it.
.
Industrialcraft is great fun. Need to check out extra biomes.
We're currently running a tekkit server but am tempted to start over with FTB. Only concern is if they update regularly, will it affect the world. my kids would get pissed if we had to press the reset button again soon. Eg FTB doesn't have RP2 yet, so won't the world be missing necessary ores?
How do you add optifine to the client? One of our laptops needs the help.
Lastly, if you're cutting out mods, would you recommend keeping the server with the same mods, or let it load everything (so you at least have the world generated with ores ready etc)