Most FAQs consider this to be one of the best custom machines:
Frost Lynx G4. Use the Aqua Goose body, Super Lynx cockpit, and Titan G4 booster. I stopped using this machine because it felt like cheating, but it sounds like most of you are gonna need it.
A little bit of advice for Diamond Cup Master; try knocking as many racers off course in the first race as possible. Since there are virtually no walls, it should be very easy, even with a light machine. If at all possible, try and make sure you take out Black Shadow's machine, and Gomar & Shioh's too. They almost always end up finishing in the top ranks, so getting them out of the way REALLY helps. One time, I managed to knock out TEN machines in the first race on Diamond Master. At the end of the second to last race, I was exactly 100 points ahead. All I had to do was finish the next race, and I won. Now that's my kind of victory.