My absolute favourite mod by far has to be Optifine, as it gives so much more control (and therefore FPS) to me when playing Minecraft. TMI is good when Modloader is installed as you can give yourself infinite stacks of items without the issue of instantly getting a full stack if you pick up a block like when using the maxstack command on SPC. Zombe is also very useful as it gives you hotkeys to most of the aspect of SPC and TMI, albeit with different usages etc (no console, key combos etc).
To summarise:
Optifine: Mod that adds many options to game setting, effectively giving the player the ability to double or sometimes even triple their FPS through game optimisation (I get about 2 - 2.5 times FPS increase on Single Player, About 1.5 times on multiplayer)
TMI (aka TooManyItems): Inventory Editor with time control, gamemode switching, difficulty switcher, and item spawning UI
SPC (aka SinglePlayerCommands): World Edit control plus many other commands to make construction etc MUCH easier for large projects. Only problem is, the atlantis command, which I gather to be useful for trying to make underwater bases, has been disabled)
Zombe's ModPack: A compilation of mods to enhance both the game and building power of the user. Compatible with most Bukkit servers (if set up so) it not only saves time but also your brain, as you only need to learn your keyboard rather than complex command strings.
That's about it from me now. Enjoy modding!
PS: I use Modloader simply to help ensure all mods run as I want them to. All it really does is stop mod conflicts etc. A variation of it is included in Optifine.
PPS: I also use mcpatcher to install my mods, as it sorts everything out in one go and lets me order said mods into and heirarchy I want, thereby letting me choose which mods should replace aspects of other mods (i.e. modloader with Optifine etc)
OK, I'm definitely finished now. *takes a huge breath and collapses from asphyxiation*