There's been a lot of talk recently about when a certain kind of writing instrument simply isn't the best tool for the task at hand. Many fountain pen collectors/users actually feel guilty if they're not using their prized writing implements all the time. But if you're in the shop, the field, or a car, a fountain pen likely isn't the easiest or the most practical writing implement. I'd venture that for most people, fountain pens work better for longer periods of writing at a desk, in a chair, etc. I'd characterize my own usage at about 60% fountain pens, 20% pencils, and 20% non-fountain pens, but my day job involves a lot of longhand note-taking and drafting, so for me a fountain pen is often the most comfortable tool to use.
That said, I still use a LOT of non-fountain pens. You've seen pictures of the various pen and pencil cups I have sitting around everywhere. Here are my favorite disposable non-fountain pens that find their way into my bag.
Pilot G2/Pilot Juice. The G2 is probaby my favorite all-around retractable gel pen, and for whatever reason I cannot for the life of me convince my workplace to swap in Pilot G2s for my most hated pen ever, the standard Zebra Sarasa (not to be confused with the Sarasa Clip, which is a great option). The Pilot Juice basically uses the same refill as the G2, but comes in a wider array of colors and smaller point sizes. Both are easy to find and exceptionally smooth writers.
Pilot Hi-Tec-C. My Japanese micro gel pen of choice. I love the needle-point tips in the .4mm size, which hits a sweet spot for me. It's small enough for annotations and notetaking, but it's also wide enough that it's not uncomfortable to use for longer pieces of writing. Sometimes you can find these in US retail stores as the Pilot G-Tec-C.
Uniball Signo 207/307. After the G2, this is probably my favorite gel-ink click pen. Uni recently introduced the "307" into stores (and finally introduced a .5mm option), but the 207 continues to be widely available so I don't know whether the 307 is a replacement or simply yet another option. Anyway, the differences between the two seem to be minimal, and if you're in a pinch in a big-box store, you can't go wrong with these or the G2.
Pilot Razor Point II Extra Fine. I keep boxes of these in my desk at home and at work. They're relatively easy to find, and they come in a few other colors such as purple and blue. These are good pens for Field Notes. I did a full review of this pen a while back.
Uni Signo DX. Doesn't have the "skippiness" problem that you sometimes find with the Hi-Tec-C, and it comes in a wide range of colors, but for some reason this pen has never spoken to me the same way the Hi-Tec-C has. If the Hi-Tec-C doesn't work for you, or if you prefer conical tips to needle-points, then this would be my recommended option. I reviewed these pens in a few of the widely available colors.
At some point I will do a list of favorite pen bodies to hold non-fountain pen refills. As you might expect, my favorite is the Tactile Turn Mover, which shipped with a .38mm G2 refill that I find awesome. Remember: Ultimately all of these "Top 5" lists come down to personal preference. What matters is that you find the pens that work for you, and that you enjoy using.