Anything and everything! A wide variety of synths gives a wide variety of sounds....

If you don't need them live, some of the soft-synths do an incredible job of sounding analog, or FM-ish, and the modeling synths can do stuff virtually (!) no 'real' synth can do... (And if you try VERY hard, you actually CAN use some of these live with a laptop)

I'm Mac-based, so my choices differ a bit from the PC crowd, but almost anything by Arturia is pretty dead-on accurate, and they have a hardware version coming out soon that can run their soft-synths in hardware. The CS80 is amazing, the MiniMoog phat, ARP 2600 dead-on gorgeous... You get the picture.

Then I'm a big fan of Spectrasonics Atmosphere.. gorgeous pads, with little CPU hit. The Oddity (for Odyssey fans). Prophet5 emulations, Jupiter 8 sims, it's all good!

My only remaining REAL analog is a vintage Oberheim Xpander, which I would trade in a flash for an OBXa, but it's still my best 'weird noise' machine! But my K2500 can do some pretty serious analog style synthesis, so I'm not too bad off!

I'm just not sure I could cope with the financial demands of keeping a large collection of real vintage synths in running order, now that parts are getting rare... But the 'virtuals' are easily able to keep me happy, even if they are not 'perfect' (No two real synths ever sounded the same, anyway!)