I'm more familiar with mods than modpacks, but I'll list a few mods that I personally recommend for building.
Chisel-offers lots of new blocks made with vanilla materials. It also has two worldgen blocks that have a variety of textures.
Capenter's Blocks- lots of new shapes, angles and texture possibilities. Plus it give us the most lacking block in Minecraft, the vertical slab. Most vanilla, and many mod blocks can be used with the custom shapes provided
Railcraft - the stone varieties are well done. You have quite a few texture and brick options as well.
Extra Utilities has the builder's wand that helps with multiple block placement. The 1.6 version can only place 9 blocks at a time. In 1.7 the creative wand can be obtained in survival mode and it has a greater range for block placing.
Thaumcraft - The Wand of Equal Trade is something I consider indispensable when it comes to mod building. You can replace almost any block with another. It's perfect for rough draft design. You can build a structure from stone & cobble just to get the shapes you want. Then you can magically replace those blocks for more suitable materials. I love this foci and it's available pretty early on in TC research.
Microblocks - I'm not a huge fan of using them for general building but they are an option. You can get smaller sizes, strip, & bits which can add lots of detail and texture to a build. I used them in a base in 1.5, but I found it tedious to use them on a large scale because they are difficult to place. They're great for covering pipes and machines so having them is an advantage.
That's what I can think of right now. I hope this helps in some way.
