The best amphetamine form is, in my opinion, consumed amphetamine.
In all seriousness: I don't think there's a huge difference in addiction potential between amphetamine (sulfate) and methamphetamine (hydrochloride). I personally have experience with these two, dex-amphetamine, 2-fluoromethamphetamine, 3-fluoroamphetamine and 4-fluoroamphetamine.
Speed paste, being amphetamine that's still in a solvent, should at all times be dried to its powder form before consumption. Some manufacturers use methanol as that solvent, which has already cost people their lives in the past. The powder form could be taken orally, insufflated or dissolved in demineralized water and IV'ed (IV'ed is actually a guess). Which is just a matter of preference. Basically, this is the active ingredient in the medication, but racemic. Also acquiring quality speed has turned out to be very difficult here in the Netherlands, judging by what results our testing facilities are getting on average. I don't know if this is the same case in other countries, but it wouldn't seem unlikely to me. If you were to get, say, amphetamine sulfate that consists of >65% amphetamine, you've managed to get a quality product and in my experience, this drug is very similar to methamphetamine, but a bit less euphoric and some other minor differences, I'll get back to that at the Meth part.
The dex-amphetamine I have experience with is the same active ingredient as spoken about above, but only the dex isomer. This was consumed in pill form of pharmaceutical grade and turned out to be the best study aid among all these amphetamines. It results in a mainly mental stimulation.
2-FMA was great as a study aid, maybe even better than Dex, but side effects showed up rather fast, which makes me disqualify this one for any purpose.
3-FA was awesome as a recreational/party stimulant drug, but my experience doesn't reach very far as I ran out faster than I expected..
4-FA is an obvious party-only drug, imo, because of its high serotonin affinity. Many people love this drug as a party drug and prefer it over amphetamine or MDMA. I personally don't.
Methamphetamine, lastly, could be considered a king among these drugs. I have heard many stories about its dangers and what not, but from what I've experienced, the exact same could be said for amphetamine sulfate, but meth deals its damage more explosively. Meth felt only slightly superior to amph in effects, but an addiction to meth could destroy a person much faster in about the same way as amph does. Therefore, I choose amphetamine over meth in general.
Then the different salts could be considered, but that difference is next to nothing.