What is the best choice is just the parts you have there are. . For example, there are outlets for 50 to 100 cc + setups. some show a bit more torque, and there are exhausts made mainly for higher topspeed. Below are some exhausts, and what they do.
Sito + basically the same as the stock exhaust, but a tad better. it's cheap, but not for performance at all. it's basically just for replacing
polini Country. probably the crappiest polini pipe out there. it doesn't really do much, but is waaaay better than stock, and better than sito+ on the bright side, you should be able to get it cheap(not here in the us though)
The malossi 28mm exhaust. Specially made for the 50cc of matching malossi.
LeoVince Curling. good low and mid range pipe. not good if you're looking for top end, it does NOT perform well. it's also made for stock/50cc kits, so past that, get something else
Polini TopOne. good top end pipe, but has a decent overal powerband. great for variated vespa setups. header is prone to rust though, and it's pretty expensive. use on kits, not made for stock engines(60cc+)
Proma Circuit. Same as the puch version, great mid range, and decent top end. pretty torquey also, inexpensive, and has a great finish, doesn't rust or tarnish easily. great pipe for stock, and kits
Malossi Too Bad. 22mm exhaust port. use for kits that have that opening size(example: 64ccMalossi)
Malossi Too Bad27mm version. this one has a bigger exhaust opening, so is for kits with big exhaust ports(27mm obviously) these too bad pipes have insane low end torque. and drive through the rest of the power band VERY well. good all around pipe. costly however
Phat Bear Exhaust. no info on this as of yet
Gianelli Excel. Goes on the other side, so watch your mounting, so it doesn't clank the transmission. expensive, but it's a top end killer
Gianelli Record. even faster than the previous Gianelli pipe.
Proma Snake. this pipe is rediculously fast on the top in, but unfortunately it's very very rare, so expensive
Proma Cannon. made for 70cc+ it's a BEAST very fast through low mid and high. race grade, and very expensive, and also very rare, and hard to get
Proma Gun, great pipe for variated vespas, 70cc+ just like the cannon, but yet even faster? also rare and costly
Proma Grandprix. they say it's more for looks, so i think what they mean is that it sux, and doesnt do much.
Proma Snake Replica. fancy, but doesn't perform like the snake. mainly for sow. it's also INSANELY LOUD.
Proma Summit. i think what they were telling me, is this isn't out yet OR they have no info on it. so i got nothing to report on it
proma Sumit. mounts on top, made for the SI. and made for bikes with big kits on them 70cc+ made for top end, like most vespa pipes.
Techno Circuit. an inexpensive alternative to the faster proma circuit. they said this pipe isn't worth the purchase. there's little to no low end power and in their words: "there is little to no boost nowhere to be discovered!"
Leovince Handmade. kick ass! plain and simple. made for kits, has a big exhaust outlet. great mid-top end power. rocks your socks off.
homoet. great solid pipe, but ONLY if you tune it. the timing of the pipe they said is off a bit, it kicks in early, so you have to adjust your timing. it's for the 50/60cc cylinders, so it's likea homoet 4p for puchs.
ninja G2. thats right frenchys we get one too!! for all sizes in our version though. (50/60/70) great pipe low mid and super high! is kinda bulky on the downside, and you'll have trouble mounting this thing to be comfortable on smaller model vespas(ciao Si)
Polini Sp. don't expect to get off the line with this pipe. it KILLS the low end. decent mid range, but a real top end screamer. there isn't really a good transition from mid to high..it just happens?? i guess if you got it, you'll know what that means
Simonini Calibretta. in their words:"this pipe doesn't ever like to walk. it's always sprinting!" the proma-gun is good, but the calibretta is better! low mid, super super fast high end. probably one of the best
Simonini Carbon. it's made SPECIFICALLY for the 100cc engine, it has a flange, so it doesn't fit on anything else.
Simonini Proline. good torque, moderate high end. it actually kicks ass from a standstill to the mid powerband, thats about it.
Simonini Racing. made for 60/70cc kits. no low end to think of, mainly made for top end power
Simonini Circuit Exhaust. good torque, but has a relatively small exhaust port compared to other simonini's (50/60ccs) top end isn't very good, but low and mid are excellent
Fabrizi. EXTREME. very very fast, and very very expensive. but most say it's worth double what it costs. not torquey but FAST AS HELL. made for kits with at least a 46mm bore. perferably something with reeds?(membrane) variated peds should get this, to take care of the low end
more to come