8086 or 8088: While IBM is a classic here, I think I remember this era better with the 8-10Mhz "Turbo" version. There were a lot made of them.
286: Kind of similar to the 8088, the 12MHz seemed quite popular
386: The 386SX 16 MHz, and maybe the 33MHz close behind
486: Pretty obvious, but there are alot of interesting ones in the family.
Pentium first series 60/66: Pentium 60
Pentium second series 75-200: Pentium 90
Pentium MMX: Hard one really, since this really was short lived era due to Intel. Pretty much any... Maybe this would be more interesting with the clones and SS7 included.
the 3 above can be merged? or at least the p54 and p55?: No
Pentium II - P2 233
Pentium III - P3 600. I feel alot quickly ended up skipped over due to MHz wars.
Pentium IV - Oh no, not the P4. Maybe something from the Northwood era. Please excuse me while I check into the P4 haters club, and somebody else can give a better answer.

shouldnt there be an "Athlon " somewhere...

This is a very region-dependent question. I'd argue that 386DX 40MHz from AMD is the most representative one, since there was a period of time when you'd either buy this, or (if you had the money) get some early 486 system. AMD flooded the market with those and there were pleny of cheap no-name mobos that had it soldered (and usually also had a socket for a 486).

For P2/P3, do we include Celeron CPUs in that? Frankly the P2, as such, was a bit expensive and a lot of people went for the Celeron, especially the later socket ones. 300A was a good overclocker for Slot1 and 400 for the socket. That being said quite a few people skipped either P2 or P3, or both, by getting something like K6-2 at 400MHz (which can work with 66MHz bus, no need for super socket 7 mobo) and then a Duron or even Athlon. So it's hard to pinpoint a representative model.

Perhaps this question should be split between business and home machines, since the former had money to spend but also certain intertia to upgrade afterwards, and the latter would be picky and look for good bang for buck rather than absolute performance.

for me its either fun ones or best sellers:
NEC V20 (fun)
Harris 286 20MHz (fun)
AMD 386DX40
486DX2 66MHz
socket 4 Pentium was very expensive and not representative of anything. It went from only servers/workstations to bargain bin in less than a year.
P120
P166MMX
P2 333MHz
P3 meh, new models released every 3-4 months
P4 meh, expensive or very bad, or both

Missing are
Celeron obvious choice C 300A
Duron 600/650
Athlon 1000

Deunan wrote on 2024-02-20, 22:52:

For P2/P3, do we include Celeron CPUs in that? Frankly the P2, as such, was a bit expensive and a lot of people went for the Celeron, especially the later socket ones. 300A was a good overclocker for Slot1 and 400 for the socket. That being said quite a few people skipped either P2 or P3, or both, by getting something like K6-2 at 400MHz (which can work with 66MHz bus, no need for super socket 7 mobo) and then a Duron or even Athlon. So it's hard to pinpoint a representative model.

For gaming the Celeron 300 and/or K6-2 were highly representative of that era. I don't think I knew anyone that had either a P2 or P3 at the time.

As you say, the Celeron 300A was popular for overclocking and touted as achieving performance comparable to a proper P2.

Around the Year 2000 AMD started eclipsing Intel for gaming rigs. Athlon and Athlon XP+ were quite popular.

Okay... here goes:
- 486DX2/66 - that's easy
- P90 (first half-affordable Pentium)
- P166
- there might be another CPU in between here, not sure what...
- oh, and there's a Celeron that should be on this list... was it the 300A? the legendary overclocker?
- PII 450
... then I can't readily think of a PIII or P4 that had the same cultural dominance as those. Coppermines were popular but I don't think there was really a sweet-spot dominant model. There was maybe one Northwood that was popular in 2003 or so, maybe it was the 3.06GHz one? Willamettes were meh, Tualatins weren't really talked about much for desktop, not sure about AMDs because I was always an Intel guy until the X2 3800+ (see my next point) almost made me go to the dark side.
(And if you wanted to get newer, I'd add Athlon X2 3800+, C2D E6600, C2Q Q6600 - the Q6600, in particular, was a CPU everyone, including recovering AMD fanboys, had... except the people, the diehard Intel fans like me, who had bought E6600s like 3-6 months before.)

8088- 8MHz
80286-12MHz
386SX-16
386DX-40
486DX2-66
Pentium-133 (P5/P54C/P55C)
Pentium II - 450 (PPro/PII/PIII)

Intel 8088 @ 4.77 MHz
Intel 80286-12
Am386DX-40
i486DX2-66
Pentium 133
Pentium MMX 200
Pentium II 400
Pentium III 1000
Pentium 4 630

8086 or 8088 - 8088 for sure. Any of the intel clones. In my neck of the woods AMD and Siemens manufacturered 8088's were the most common.
286 - 8 or 12MHz seem to be more wide-spread
386 - 33MHz SX and 40MHz Am386
486 - 66MHz DX2 and 133MHz Am5x86
Pentium first series 60/66 - none. These are rare. They're rare now, and they were rare back in the day.
Pentium second series 75-200 - 120 and 166MHz.
Pentium MMX - 200 and 233Mhz. Surprisingly common, I believe the 233 served as a cheap alternative to pentium 2 machines
Pentium II - Most common P2 chips I've seen are the 333 and 350MHz. More common then that, the 300MHz celeron.
Pentium III - Slot 1 P3's were pretty rare in my neck of the woods. Mostly reserved to OEM PCs. 500 and 550Mhz were the most common. For skt370, by far the most common chips sold here were the 933 and 1GHz coppermines - like with the Pentium 1 233, I believe P3's were sold as a cheaper alternative to pentium 4 PCs.
Socket 478 Pentium IV - 1.7Ghz and 2.8Ghz are the most common models I've come across, but the most famous has to be the 2GHz, as I seem to remember them appearing in lots of tech publications back when I was a teen.
LGA775 Pentium IV - 3 and 3.2 GHz

Why only intel chips tho? Here are some AMD ones.
386 - Am386 DX40
486 - I've previously mentioned the 5x86-133. This was very popular in my neck of the woods.
Socket 5/7 - AMD K6-2 was very popular. Particularly the 350 and 500MHz models.
Socket A - Duron 750 and Athlon XP 1700+. I think I have a box of these someware. For the later part of socket A's life, I'd say the 2500+ barton.
Socket 754/939 - that's easy - Athlon 64 3000+ (754) and 3200+ (939)