Currently the only mod that I didn't make that I could recommend is Optifine; otherwise, I'd recommend my "old caves" mod from the second link in my signature, which restores cave generation to the way it was before 1.7 (average cave system size was reduced to about 36%, 1.7 also reduced mineshaft frequency to 40% of 1.6.4, though those were too common anyway, at least far from the origin where they are more common). Of course, even now some people complain there are too many caves.
I've also previously used the following, in 1.6.2:
Rei's Minimap Backpacks (near infinite inventory space! Although a bit too OP for my liking, as is the case with many mods; when I used it I used four big backpacks for ores, spending up to a real-time day on a single caving session) Amethyst Armor/Tools (see Backpacks, particularly due to the damage the sword does; I actually bytecode edited it to nerf the damage/balance out armor durability with tools and disable the crafting recipe so I could add an ore)
(my mod "TheMasterCaver's World combines aspects of these mods, in more balanced ways, such as using a modified Ender chest instead and in a way that discourages using it as a backpack, instead using a normal Ender chest for that purpose; the amethyst tools/armor is only increased in durability and is harder to find and enchant than diamond)