In reply to andycamach's question, I use an OOPic (obviously), and I've got a 68HC11 controller I built years ago that I'm about to put back into service. The next flavor I'd like to get into is the Atmel AVR series. They're inexpensive, powerful, and have nice development tools. Can't beat it. But I'm not going to get rid of either of the other two.
On another topic, I ran face-first into the whole idea of a wire gauge limiting the amount of current you can move through it. I replaced the tether on my ROV with a smaller gauge, more flexible tether. Voila I lost half the available power. DOH! I swapped back to the beefier but stiffer tether.
I think I'm running into a fundamental limitation of small ROVs. A small motor can still draw a lot of current, so the wiring still needs to be beefy. So you're still dragging around a thick tether. So you need beefier motors. So... There's a point of diminishing returns.
In an ideal world I'd put the batteries on-board and not run a powered tether at all. An RS-232 or RS-485 connection would make for an extremely lightweight tether, but it would require brains-on-board as well. At that point I'm out of the micro ROV end of things and would do better to go ahead and build a larger platform. That's for the next ROV project.