depends on budget and insurance budget/cost. i was looking at honda cbf125s 1700 for a 2012, not gunna win any races. but should be fast enough for most things honda cbr125r would be pretty rapid, but insurance would be alot more in comparison im running a chinese cg125 rep (damn close) cost me nothing (2003 plate) and insurance is dirt cheap. but its not fast (faster than my old 50 ped) it will happily get to about 50 before acceleration falls off. and so far ive had a top speed of 63 (rubbish)
Might wanna check the lineage of the engine of anything you have taken a fancy to - as it'll give you a rough idea of what the manufacturers had in mind 1stly, Some may be older designs tarted up!,reliability may come into ?, Some may be more specific / newer an aimed more towards performance. .
as above depends on your budget and style. I've got a YBR 125 and had a CBF125 they both great bikes. I was highly recommend the CBF due to it's MPG and can handle the higher revs better then the YBR but if you want sporty then the CBF is great to. (depending if you're looking for 4t)
all the 125 bikes are the same really, OHC 14hp top out at about 70mph. at a guess the new rs125 is probably pretty fast, but like all the others its gonna be restricted to 14HP. the cbf is a good bike as is the cbr (old and new shape). very easy to handle and very light. if you buy a good used cg or something you could probably hand the motor into a tuning shop (or do it yourself if you know what your doing) and get it ported and such. another option is to get a motocross motor in a road bike frame (but be prepared for high maintenece)