I don't know if you finished the game yet, but if you haven't, I suggest that you do so before adding mods, just so you have a good feel for what the vanilla game is like before you start changing stuff... it'll also give you a sense of what you don't like or would want more of, and that'll let you more intelligently pick mods.

That said, I'd divide mods in to several types:

First, there are convenience mods, which make certain parts of the game less annoying or less tedious. Among those, I'd recommend:

- Squeak Through - Lets your character squeeze through spaces that you couldn't in the vanilla game. It doesn't sound like much, but it make a huge difference and makes the game much less annoying.

- Tree Collision - Similar to Squek Through, this lets you move through forests easily.

- Splatter Guard - Keeps you from getting killed by trains.

- Repair from Inventory - Automatically repair items in your inventory.

- Packing Tape - Lets you carry filled containers.

- Fill4Me - Automatically fills things things like turrets with ammo when you place them.

- Even Distribution - An easy way to put the same amount of something in to multiple entities (ex: same-sized batches of coal in furnaces).

- Belt Reverser - One keystroke to reverse the direction of a belt. Much easier than doing it by hand.

- Atilla's Zoom Mod - Zoom out much further than you can in the vanilla game. Also has keystrokes for quick zoom in/out.

- Todo List - I couldn't live without this mod, which lets me keep track of what I need to do.

- Far Reach - Lets you manipulate far way items as if they were near by. This could be considered "cheating" by some, but it makes the game much more convenient. If you are concerned that the game is too easy this way, you could always raise the difficulty with other mods.

- Where Is MyBody - When you die a line will be shown on the screen pointing to your body, so you can find it more easily and get your inventory back.

- What is it really used for? - A nice way to find what items are used for (which the vanilla game does not tell you.. it only tells you how to craft items).

The second category of mods is one that adds major features to the game and/or lots of different items. In this category, I can reommend:

- Factorissimo2 - adds "factory" buildings that can contain other buildings (including, if researched, more "factory" buildings). This is very useful for packing a lot in to a little space. If you're constantly running out of space this could be a good mod for you.

- Bob & Angel's mods - if you want a trillion intermediate products to extend the game

- Yuoki Industries - lots of strange, different items and intermediate products that feel very different from vanilla

Finally, if you found the vanilla biters too easy and want more combat, I'd recommend the Rampant mod, which makes the biters smarter and tougher. If you find Rampant too hard, you can dial down the difficulty in its options and/or install some other mods that give you more powerful weapons.

Ok, so this wasn't a small list of mods.. but I couldn't narrow it down any further without feeling like I'd left out some crucial mods. Hope you find something here that interests you.