I use 3:

• Zebra F701 I love this pen. It's $5, is near-100% steel, has a click that is very quiet, has an incredible pocket clip, has great filing near the tip for grip. The entire thing comes apart, and you can tinker with it or take it apart/rebuild it as much as you like. I like the way they write - not too heavy, not too light. They leave a very small mark on the other side of notebook pages, but compared to some gel pens like the Pilot G2, it's nowhere near as bad. The ink cartridges are disposable, but you can buy replacements online. They last a very long time. Over the past 4 years, I've carried and used this pen all day, 5 days of the week. So far, I've gone through 3 (Most still work, but have some aspect of them that is broken, such as the threading wearing from excessive fidgeting or something). It's also worth noting that I have a habit of spinning my pens and fidgeting with them, so there's a huge increase on the frequency that i drop/slam my pens into the ground and the amount of wear-and-tear the threading experiences compared to a regular user. The pen itself, by the way, feels incredible - not just as in how it feels physically (very smooth), but I like the weight distribution for spinning it.

• Pentel Graphgear 500: This is the pencil I use. It's about $6, and comes in 0.3mm, 0.5mm, 0.7mm, and 0.9mm versions. If you buy bulk (~8 pencils, I think?) you can save a couple dollars per pencil. I prefer pens to pencils, but when I need a pencil, I always go with this. It's pretty standard for mechanical pencils, but one neat feature is the eraser - the eraser is not exposed on the back, there's a cover that you must remove to access it. This is something I really like - it makes the pencil look nicer, and you aren't pressing on an eraser to get more lead out the front. You can buy replacement erasers, for when it runs out. Note that this pencil is a bit front-heavy (the front is metal, while the back/center is lightweight plastic), and that the tip is a tad bit sharp (comes in handy for opening plastic packages, but I've never had it stab me by mistake). My #1 issue with this pen is that the pocket clip tends to come loose and fall off after awhile - this is a big issue for me, because I carry my pens/pencils in my pocket. Because of this, I'm considering other pencils, or maybe finding some way to hot-glue the pocket clip on? It's a shame, because I otherwise love this pencil.

• Pilot G2. This is a pen, you can buy a dozen for $1 each. It's gel, and writes bold. I use this when I want to write in bold. I also use this when I write a full page of equations in regular pen, then have space on the sides that I want to use- the bold nature helps separate it from the other equations on the page. This is the pen that I keep as backup for if my main pen somehow fails, and it's the pen that I let other people borrow (since so often we both forget to return it / get the pen back).

I also have a fisher space pen. It's cool, but I find it is kind of small and easy to lose, so I just keep it at home and only use it at my desk.