Depends on your "budget" and how much work you want to do. If you're trying to keep it cheap, under 1500 bucks, a '70s Honda CB or other Japanese bike. Between 2-4000 bucks, lots more Jap bikes and maybe a British twin. If money isn't a problem, then you've got all kinds of choices. What do you want to ride? How much work can you do?
For cheap, I'd look real hard at Yamaha XS650s. There are tons of parts available and the bikes are cheap and reliable to start with. If I ever get my GT550 done I would like to do an XS streettracker/caferacer/hooligan bike. I'd stick with a four stroke if you really intend to ride it alot. The prices for popular vintage two strokes are crazy.
I am a big fan of suzuki's T500 - brit bike chassis layout, dependbile japanese two stroke 500cc twin. They are kinda getting expensive these days but they can still be had for reasonable prices.
I always recommend a cb750 but you don't want an across the frame four.
Check out the s3 kawasaki 400 triple. Best handleing 2 stroke triple ever made and the most reliable. As fast as the 500 but much lighter and more agile. If you buy one make sure it is running as NOS pistons cost more than the bike itself.
cb450 is a really nice choice also, easy to break the ton with a light enough rider.
a new enfield 500 cc single is always a nice choice after you replace all the indian parts with brit parts.
GB500 is a nice bike too, all the hard work is done for ya ,cafe out of the box. Expect to pay.
really there are so many choices you need to be more specific about how you intend to use the bike.
Well, I'd like to get out to the track and run it there mainly... a little fun around town some times, or out on a twisty road, but mainly track riding. The 2-4000 range sounds about right and I can do fabrication on some parts. I like the lookof some of the Brit. bikes(looking at the BSA's and Triumph's), but I don't want to deal with the shifter and brake swapped, I have a 998 Duc and I'd hate to shift down when I meant to brake, it would probably be a costly mistake. Is anyone making rearsets and linkage to swap them? The XS650 sounds like it might be fun I'll take a look. Thanks for the responses!
I'm only familiar with Jap bikes and some of the best street/track cafe conversions I know of come from the CB 350/450, XS650 or even the SR500 single. Same reasons stated above: reliable and cheap.
Didn't some of the American release Triumphs have the "standard" brake/shifter setup?
If you go with the XS, go early. I seem to recall the later ones were dogs. I've had personal experience with two XS's and on both of them the gearbox was shit. Swapped a boneyard bottom end in both and they were fine...