I've seen this sorta discussed in off-topic fashion in other threads, so figured it'd be a good exercise to centralize it here and discuss. Exercise: name a watch brand and its fountain pen analogue and provide a rationale as for why. No right or wrongs here (aside from mistakes in fact). if anything, it might be useful for newcomers too.
WATCH><PEN : Rationale
ROLEX><MONTBLANC: the most well known / cited brand for their respective luxury categories
SEIKO><PILOT: both Japanese (obviously), have reliable quality products in a large range of prices (from super affordable to high end, i.e., Grand Seiko><Namiki), in-house movements/nibs
CITIZEN><SAILOR: again, both Japanese but neither not nearly as ubiquitous as SEIKO/PILOT at price tiers nor offerings though both have products in the very affordable to the high end range; in-house movements/nibs (and proprietary converter..grr).
ORIENT><PLATINUM: you guessed it, both Japanese. Reasons similar to CITIZEN><SAILOR, but my thoughts for this comparison is that ORIENT & PLATINUM are lesser known by the general public than SEIKO, CITIZEN, PILOT and SAILOR
NOMOS><PELIKAN: both German and reliable quality products. neither has anything in their relative, very affordable price range, but start around entry luxury tier for their respective industry to the higher end, both use in-house movements/nibs (and pistons) [though, NOMOS pens are produced by KAWECO]
LACO><KAWECO: both german, offer products at range of price points and many variations in styles (considered STOWA, but LACO is more equivalent in aesthetic diversity to KAWECO than STOWA).
TIMEX><CONKLIN: storied American companies that offer affordable products for everyone. both can be found in retail stores not specializing in watches/pens (TIMEX..well, most anywhere, CONKLIN in office supplies stores like Office Max and Staples). both endorsed by Mark Twain
There are a bunch more I missed, but that's enough for now. I'd be interested to read others' comparisons. What of, say, DANITRIO, VISCONTI, LAMY, PARKER, SHEAFFER, TWSBI?