I'll second the 8500/9500 models. But the P.C. Power Tower Pro was tricky, too. As awesome as that case design was, you had to install most of the RAM sticks by feel since you couldn't see the full length of the slots - kind of like changing the spark plugs on my Firebird.
My vote would be for the old compact Macs like the SE/30. Not really because they were that ultra-difficult to dismantle - as long as you had the right tools (case cracker and special screwdriver) - but the whole exercise was so fiddly. You had to remove the motherboard completely, then there were all kinds of rules to remember about cutting or unsoldering jumpers/resistors and putting the RAM in by banks, then the slots had delicate little plastic latches that were ultra-easy to break. Then, if you got one of the eight SIMMs in slightly crooked, it would be "The Chimes of Death®" and everything to do over. A G4 is laughably easy by comparison.
Another dandy is the Power Mac 7100 - I still have one of these beasts. To seat the RAM or the motherboard battery properly you really need to dismantle the whole computer, Those with tiny fingers and a positive balance at the blood bank can do it without dismantling, but not me.
You are thinking of the MacPlus not the SE 30. Resistors needed to be clipped to go above 2 megs of RAM and resoldered to go down.
Yeah that was a nasty - SE30s a piece of cake in comparison - lots of slots
The 9500 is the worst without a doubt. If you have all 6 PCI slots filled you have to remove every card as well as the CPU card. Then you have to remove all the cables from the motherboard. Then you have to remove the motherboard. Then you install ram and hope everything goes back together again the way it's supposed to. Apple was really thinking different back in those daze. (I have two of these bowsers and I get chills just thinking about the number of times I've had to do this)
What's even worse is that you have to go through this same nightmare to replace the pram battery.
The same stupid design was used in the 8500 too, but since it only has 3 PCI slots it was better by default.