I've owned Fieros since the late 90s and have had an opportunity to use many aftermarket parts. I have had good experiences with a few brands and I'd like to pass this on. Hopefully others will join in And post their experiences as well.
For oxygen sensors I only use the Denso brand they appear to work perfectly and last.
Bosch products like water pumps and fuel pumps work well and have proven reliable.
For brake parts Centric, Raybestos and Wagner have worked well.
My choice of ignition wires has been MSD Superconductor wires.
Wells distributor caps and rotors seem to work ok but they may be just reboxing whatever they can find.
AC Delco products including ignition modules that come from Mexico have worked but Buicks are now made in China. Wonder what components they use.?.
Delphi seems to have a better quality control system than most of Chinese automotive replacement products but the are expensive.
There are certainly more but I will post them as I think about it more.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

My aftermarket parts experience (both Fiero and other GMs) have me agreeing with you across the board. In addition to your list, I also like Standard for a lot of parts, switches, sensors, and others. Airtex has done well for me too, although most of those were not on Fiero. For distributor caps, I always look for brass contacts - so many are aluminum now. I've bought a Standard one that looks good (haven't used it yet), and a BWD/Niehoff that has worked well (I know BWD has a less than stellar reputation on some parts). But the brass contacts aren't my only criterion ever since I had an AC Delco cap where one of the contacts came out when I pulled the plug wires.

For water pumps, idlers, and stuff, I've always had good luck with Gates, although sometimes they're more expensive than other perfectly good brands. I like some Bosch parts, like the alternator that's been flawless on my '88 GT for over 7 years now. Word on this forum is that their O2 sensors suck; not sure I feel that strongly about them but they haven't been good to me either. I've had good experience with Cardone parts too, like the new (not rebuilt) distributor that's been in use for about 5 years now.

I changed my O2 sensor yesterday (trying to pass emissions tests here in suburban Denver CO) . I wanted to take the advice of several on the forum who swear by Denso, but the part number I found for the V6 is 234-1001, and I can't find stock on that anywhere. Hard for me to believe since so many cars used 1-wire narrow-band sensors. I finally bought a Delphi, but when I received it I found that it looks a lot like the Bosch, and if it isn't the same, it appears to be a similar design. My AutoXray OBDI scanner says it's working okay. Would NTK be better? Is the Denso available under another number, maybe without the connector? Incidentally, it did not help my emissions - still failed with high NOx after verifying the EGR system, but that's another story.

Anyway, those are parts I've had good experience with - as always, YMMV.

You Get What You Pay For - Avoid 'Cheap' Aftermarket Parts!

I was a ‘fan’ back in 1983 before the first Fiero hit our local dealer showrooms, and I’ve owned three models over the years which my last is the '88 GT.

To me, I still try to stay away of aftermarket parts if possible and look for Genuine GM parts instead.

ACDelco is the 'best' aftermarket parts backed by GM and I search NOS parts which often the package says "Make in USA"

So to me, I look for authentic and quality, NOT the price of an imitation.

However, many consumable parts are only found in aftermarket these days, and many are from China.

As Dennis LaGrua pointed out, these 'better brands' will cost more but they'll last better because they undergo more stringent testing than the 'cheaper' aftermarket products.

Or pay less and replace it more often.....

------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles