Though I have a rather extensive pen collection by most standards (100+ pens, I haven't counted recently), I have three that have pretty much become my standby pens that come with me everywhere I go. I'm sure I have different preferences than you might, but here's at least my reasons why my favorite pens to carry around are the LAMY 2000, Pilot Custom 74, and Pilot Metropolitan.
This is an understated yet elegantly simple pen. It's large ink capacity and ease-of-use piston filler make it good for writing for slightly longer sessions, and the snap cap make it more convenient to use for quick notes than other pens that you need to twist to uncap. It's not cheap but it is a good value for the build quality, writing quality (14k gold nib), and balance of the pen. It's no wonder this pen has been a mainstay in LAMY's lineup for nearly 50 years.
I use mine with a medium nib because I like a little shading in my ink, but I also appreciate the finer nibs, especially when working on cheap paper. The pen has a reputation for having kind of a narrow 'sweet spot', and that's most noticeable on the fine and extra-fine nibs. Check out our video discussing this here. For that reason, a few people just don't like it, and that's okay. But for the ones that do get the hang of it, this pen easily becomes a staple in their pen lineup, just like it has in mine.
At the time I started using this pen, it was the most expensive pen I'd ever owned. I love blue, so that one was an obvious choice for me. But what really sold me on this pen really had nothing to do with the features, color, design…it was all about how it felt when I wrote with it. The 14k nib has just a bit of softness to it, which I often call 'spring', that acts kind of like a shock absorber when I write. It makes the already smooth nib just glide across the page and gives me a responsiveness in my writing that is hard for me to explain except by saying that this pen just 'feels right' to me.
It is cartridge/converter, but it uses Pilot's Con-70 which is the largest converter they have. It's a bit of a pain to clean when changing colors, so I tend to break my normal habit of changing inks every 10 words (or so it seems) and I seldom change what I keep in this pen. In fact, with rare exception I have only used Pilot Iroshizuku Kon-Peki, Noodler's Blue, Noodler's Liberty's Elysium, and Noodler's 54th Massachusetts. Liberty's Elysium is my mainstay these days, especially because it so perfectly matches the pen! That's something I never used to care about, but has been more important to me in the last year or so.
So this one might throw you off a bit, as the other two pens are much more expensive and have gold nibs. Why the Metro? Well, it's just a workhorse. It's solid, writes well, holds a decent volume of ink for a cartridge/converter, and is just a great knockabout pen. I throw it in my bag, my pocket, whatever, I don't care. I just beat it up because it can take it, and since it's inexpensive, it's not a big deal if I scratch it (haven't yet), drop it, or lose it. This is one of the first pens I recommend for new people getting into the fountain pen hobby, because it's an incredibly good value and it is a great introduction into what fountain pens have to offer. There are a couple of compromises with this pen. There's no ink window so I don't really know when I'm about to run out of ink. But despite all that, I love this pen and keep it inked with either Diamine Red Dragon or Diamine Ancient Copper.
My experience is my own, and though I've objectively used many, many pens for reviews and work purposes, these three pens have been the ones that for me have stood the test of time and continue to have a dedicated place in my laptop case. But it's not only limited to these three, I often carry at least four or five, rotating out the newest pen I just got in, or something particularly special to me. But my LAMY and Pilots are the ones that are always there, inked up and ready to go.