My first regs were from Mares. It wasn't for any particular reason other than they were a good deal from the shop I happened to walk into. My desire to learn more about regs in general naturally started with what I had and over time I collected more Mares regs and the tools, the parts and the knowledge, especially via rsingler's class, to work on them.
But frankly, it's been a PITA to work around a manufacturer that really doesn't want individual divers to ever open a reg and doesn't interact with users except through marketing releases. It's Mares in my case, but in the USA at least the same applies to ScubaPro, Atomic and AquaLung. Because of this, if I had to start over knowing what I do now, I'd definitely go with Deep 6.