I would have to say my 21" AA Bash ride. It works in every style I play. For me its the perfect blend of wash, ping, bell, and crash. If I had to, I could play a whole gig with that and a pair of hats as my only cymbals. But my 19" AAX Xplosion fast crash makes for a great pairing.
As much as I love the sound of darker rides from Meinl and Zildjian, I can't find a ride I feel would be worth replacing it with.
Walked into local guitar center (with my new girlfriend) to buy a pair of drum sticks. Walked past a drum set which caught my eye. Grabbed a stick. picked up one of the toms and hit it. Beautiful music to my ears. Long story short, $3000 later... I walked out with a 5 tom, 22" kick Yamaha Absolute Maple drum kit with a Yamaha Jimmy Chamberlin snare. 12 years later, I've still NEVER regretted the purchase. My (now) wife still remembers it and thought I bought it to impress her. I didn't, she just happened to be there. Best sounding kit I've ever heard... to date.
Can I say that my best drum gear investment has been my drum set? Because it motivates me to play just looking at it, it's exciting and inspiring to play, and it also fulfills a lot of lifelong wishful thinking. That's definitely my best drum gear investment.
If I can't say that, I'll say either the Tama Rhythm Watch or my old Remo Practice Pad (but these kinda go together.)