I went from the top down, for a few reasons. Here's the order in which I got stuff:
0) Fins, snorkel, mask, gloves, boots, hood (Got these when I was going through certification. These are personal items, it's just best to own them)
1) Regulator and basic cluster
2) BC and lead
3) Light, knife, slate
4) Wetsuit
I pondered it and ended up going with the reg and bc as my first large purchases. They're very personal and can have a real effect on the dive. It's nice to know I've got a reg that I know is well-maintained, and I like knowing that I have just the weight I need in the configuration I want on a BC that I know fits perfectly. I've seen many a rental BC with tears in the weight bags, leaks around the inflator hose attachment, and rips in various places. I got a basic console with an SPG and pressure gauge, and plan to purchase a computer down the road, once I've done some research.
The knife, primary light and slate came next, as I started diving more. The knife is wonderfully handy in kelp, and as a general tool and the light is great for evening dives and looking into hidey-holes in rock reefs. They were small purchases, but worth it.
I've saved the wetsuit for last for a few reasons. One, I was and still am losing weight. Two, my LDS takes good care of their rental equipment, so I've never got one that was torn, stretched or reeking of urine. Three, wetsuits are very cheap to rent always seem to be on sale. I've also got to dive in a few different brands so I know which fit I like best.
Perhaps your situation is different than mine, but I found that this purchase plan got me good gear for my money and has saved in rental costs over time.