(Warning! I wrote much more than I was expecting to)
What is it that you're looking for in a keyboard? Mech keyboards are probably the only piece of hardware I follow. I've got a horrible habit of collecting them (I actually need to sell some, but I'll get to that one day)
Whenever somebody asks me about which keyboard to buy, I frequently recommend the GH60 or KC60 if you're unable to find the original. As it happens, the KC60 is up on Massdrop right now, and it's being sold at a very reasonable price. I recommend it so often because it's more customizable than any other keyboard. You can swap the firmware for TMK if you like, the switches can be replaced without soldering (if you get the non-LED version), you can add more layers than you could use on the board, and it's compact and cheap.
The biggest feature of the GH60/KC60 is something you just don't find on other boards. You can change the layout to practically any layout imaginable. I'm not talking about changing the keycaps, I'm talking about changing the location of the switches. Want to add a split-shift on both the right and left sides of the board? No problem! Want to split your backspace key into two separate keys? Easy! Want to change the bottom row to winkeyless spacing? Sounds good! It's an incredible feature, and it's why I bought the board in the first place. There are very few boards that have this feature, even today. The few boards that do have taken inspiration from the original GH60.
Those are the pros to the board, but keep in mind that it's something you'll need to get used to.
The cons would be the size. It's not a con for me, and it doesn't lose any functionality, but it's certainly something that takes a week or so to get comfortable with. Using the arrow keys or numpad is no different than typing a capital letter (eg; Fn+W/A/S/D for arrows), and you can change those settings to whatever you like. I keep either esc, ctrl, or Fn on my Caps Lock key, and use SpaceFN for my main function layer. With that functionality, it makes the switch from a full-size no more difficult than learning to type a capital letter (Shift+X).
If you're not looking for a full-sized board, the CM Storm (Coolermaster) and Quickfire Rapid are both very popular TKL boards. The Quickfire Rapid can be hacked with the Frosty Flake controller, which is actually pretty neat. The guys over at 1upkeyboards sell them, and from what I understand, they turn your QFR into a fully programmable mech, so you can load TMK and do whatever you want with the thing.
I prefer recommending smaller companies, though. There's a great up-and-comer right now called the Clueboard. It's custom built (and pretty inexpensive) as you order it. You have control over much of the board as you select the parts used, but you are forced to use their layout. it's an interesting layout, and I've been meaning to get my hands on one, but I'm broke! Anyway, I wanted to throw them in for good measure.
There are lots of other boards coming to mind, but I feel like I've been ranting long enough. Basically, try and go for a board that allows you to load TMK. Fully programmable boards are absolutely worth the effort, and they'll certainly last.
Oh, and my last suggestion would be to make your own board! It's getting cheaper and cheaper to throw a teensy into a home-made (or ordered from a laser-cutting service) acrylic case & plate, you can even order laser cut aluminum. You don't need to design a PCB, you can hand-wire the board, and you can load TMK for full functionality. It's a fun project, but probably not for everybody.
Let me know if you have any questions, I'm quite the novice when it comes to gaming hardware and all that jazz, but I'd like to think I know my keyboards.
Edit: Bad grammar. Still getting used to this text-box format.
