Uhmm...
Most recent one has to definitely be the SSD.
Also not so much 'hardware' related but moving from 1 monitor to 2 I can now not see how possibly I could have lived with just one monitor for so many years.
Seeing as others also mention some older stuff I would also add my biggest WOW factors.
Moving from B&W IBM 16mhz / commodore to pentium 1 100mhz and 16mb of ram and 8mb vga gpu was definitely something revolutionary (and a cd-rom in times when games took no more than 40mb space).
Obviously dial up > LAN ... WOW 20Kb download speed

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3dfx vodoo.
radeon 9800.
Playstation 1 - this was a huge WoW, really it was amazing how long this platform has lasted and how good the graphics and games were at the time.
CRT > LCD.
This is a big one, moving from £3.99 optical mouse to Logitech G5 ( costed me like 70quid at the time but OH GOD THE WOW WHAT IS THIS HOW IS IT SO PRECISE AND QUICK ).
First mobile with colour lcd screen, was SEt68 I think, cant remember well - and the attachable camera, craziness.
First set of 5.1 speakers and playing FPS with it.
First MMO (UO) - what do you mean these guys are all players from around the world?!!?
World of Warcraft when it first came out, amazing world and experience like no other game could deliver before.
Move from Athlon 64 to OCed c2d E6600 (WHAT DO YOU MEAN IT HAS 2 CORES!? lol..) - move from c2d to quad had some wow in it but I don't feel the difference was that huge anymore, dual core just changed the entire world of multitasking to me, you could actually now play a game while having other things running in the background without worrying about dropping fps!.
First smartphone ( p990i ) - wow the screen quality back then, and the camera.
Symbian (SE Satio) > Android (HTC Desire) - there are no words to describe the difference in how much smoother and easier everything was ! Totally different world.
And lastly I would just add that moving from in-ear the proper over-ear headphones (Goldring NS1000 / Beats ) was probably one of the best things I've ever done in my life.