I would have to say heroin. First opiate I tried, and it's one of the easiest for me to obtain (besides percocet and hydrocodone, which are too weak.) Just the other day I obtained some IR oxymorphone, and oxycontin. Oxycontin is a good high IMO, but it lacks the rush that heroin gives you when injected, and it's just way to expensive. I would say that a 40 of OC gives me the same effect that 1 bag of the dope I can get. Now oxymorphone... that ones a monster. In all honesty, I would probably choose this over heroin, but it is just too hard to come by. I'm seriously shocked that it's legal though. The rush from just 2.5 mg of opana feels like a 2 bag shot of dope with 0 tolerance. IMO, it's actually more intense. The only come down to Opana though, is that the high doesn't last as long as dope.
I've never been able to get high off of codeine. After 9 months off of opiates, I used 15 mg of hydro, and I have to say, that was a great high. Very clean feeling, not too sedating, great energy. Problem is, half of my addiction, is to the needle, and you can shoot the hydro, and then obviously, it's very weak too.
Methadone and bupe are strange IMO. Both ARE capable of getting you seriously fucked up, but if you're on them for maintenance, they both lose the ability to produce euphoria. At lower doses of I.V bupe, I do get a noticable lift out of it, and prefer it to the done.