lately, it has been AMD. the athlon is inexpensive and more or less the same speed as an equivalent p3, but the athlon is available at 1.2 GHz whereas the p3 is only up to 1 GHz. the athlon is usually faster than a p4 at 1.5 GHz, but is slightly slower in quake3. of course you can build a whole athlon system for what you can buy just the p4 processor for. right now multiprocessing for athlon is unavailable, so the p3 is better for that.
edit: intel chipsets seem to have less issues than chipsets for amd products. but thats a platform issue, not the processor itself.