For my 15th I managed to get something that for me is VERY special: a Matchbox Sonic X 5-Pack that was only released in North America in 2004/2005. It's still mint and in the box!

These are the 5 vehicles available:

2000 Pontiac Piranha/Sonic
Baja Bug/Tails
00-01 Nissan XTerra/Chris
99-06 Chevrolet Suburban/Knuckles
95-99 Ford Transit/Dr. Eggman

Additionally, as part of a Convoy reboot, a Sonic X Tractor Trailer was also issued.