I don't think any Pentium with a 4 after it is going to be a popular retro rig.The performance is too low relative to a comparable AMD chip and the heat output is too high. The Athlon/Athlon XP/and Athlon 64 will be popular.
Socket 423 2.0Ghz Willamette with RAMBUS! 😁
And in the far future, I'd say a very wanted retro rig would be the Conroe 😁
And it will be either single core or non-single core retro rig? 😜
Edit:Otoh, when looking back from the future, P3 is the winner compared to Thunderbird, especially if you consider that any Pentium 3 will run cooler then most Thunderbirds (and on top of that, P3 supports SSE while Thunderbird does not).
Theres 2 drawbacks for P3 though:
-Even though the Intel chipsets are basically the best ones to get, they are hurt by the artificial 512MB RAM limit Intel placed upon them and the only way around that is by either using a RAMBUS P3, going BX or by picking a non-Intel chipset.
-Thunderbirds can be unlocked with a pencil 😁
Comparing Conroe to A64, I'd say Conroe is the winner here. It's faster, has the Intel chipset and runs a bit cooler (especially if you don't go the Venice route). Imo the A64 boards prior to AM2 were really a step-up compared to XP.
One advantage A64's do have is the good availability of AGP boards and the fact that (as the Intel boards can be damaged due to their onboard pins) the AMD boards are less likely to be bought second hand damaged