Yeah, yeah, I know about the super popular Inka keychain pen, but have some reservations about it (after spending a few days reading online reviews of it, as though it's a luxury vehicle or a major appliance). Instead, I opted for the following, shown left to right in the picture: Zebra F-301 Compact ballpoint; Zebra Minna Mini ballpoint; Zebra SL-F1 Mini ballpoint; and Zebra Penpod ballpoint.
The F-301 Compact is based on the regular F-301, which is good for a ballpoint. It has a lanyard hole to accommodate keychain carry, but I think it's too long to keep with my keys in my pocket. When closed, the cap is very secure, but when posted on the end it is really wobbly and shaky. When the cap is posted, the pen is virtually a full sized pen. This makes it comfortable to write with, but an EDC pen for me will typically be used only for quick notes and reminders. All of these factors put the F-301 Compact in last place for me.
The Minna is super thin and compact, and I like the design even though it's a little feminine. It might be tough to see in the picture, but there is a very cool Japanese style graphic in a clear window section of the barrel. The body is very thin, and most of the pen fits inside the cap when closed. I like how the ends are rounded and the thin body makes this easy to keep in my jeans coin pocket.
I'll take the SL-F1 Mini for the win here. This pen is weighty (I believe it has a metal construction), compact, and sleek. The gray color is nice, and it does come in other colorways. The pen telescopes to reveal the tip and to extend the body. The extended length is somewhat short, but still OK to write with for a quick note or two. When closed, it fits perfectly in my jeans coin pocket. Moreover, pen has a deep carry clip configuration, which I like better than the shallower clip found on the Minna.
The Penpod is a good option if you really want to carry something on your keychain. I don't, and the girth and deployment mechanism of the Penpod make it less than perfect for pocket carry. The pen itself is spring loaded and a quick twist releases it from the cap, which stays on the keyring.
So I wasted a few dollars and a whole lot of online research time to realize that the Zebra SL-F1 is nearly perfect for my needs. It usually lives in my jeans coin pocket (or in some other pocket when I'm not wearing jeans) and it's so small that I don't even notice it.