I use Archie Braddocks flavourings and have done for years, it's all down to personal choice really and trying them out and what results they bring. It also depends on the species of fish you are after as well. Some flavours work better in summer and visa versa. You can check his site out at www.braddocksbaits.com.uk and see what you think. 
only tried one of the rod hutchinsons flavourings, a strawberry one, did quite well, especialy with the tench, resulting in a p.b. for my dad, 7,12. from a water that hardly ever produces tench.
One that I can reccommend (but I don't know if you can still get it) is Catchum, I bought a bottle of scopex five years ago and its still as potent today as it was when I bought it. And I mean POTENT! I keep it with the lid down tight, in a jam jar with the lid down tight and inside a sealed plastic bag, and you can still smell it!
six drops is plenty for a two pound loaf,(incidently, my wife goes crazy if i open the bottle within ten yards of the house, but, when i'm bakeing it in a loaf theres no complaints)
also used it for maggots and boilies, it works.The only other flavourings i'we tried are supermarket stuff, marmite, vanilla, almond, various sauses, I like to try different things and they all seem to work, to some degree.
As said above, a lot of it is down to personal preference in this case as to whether you believe they work or not. I believe that there is no harm in flavouring baits but I very rarely flavour all my baits instead preferring to use natural baits with a few flavoured hookers/loosefeed items to make them stand out.
The exception being Archies Maga Turmeric powder which I use a lot in winter when fishing for roach in local stillwaters, its very good stuff and it goes on all my maggots/pinkies.
It is exactly this unanswerable question that makes fishing so great.
You can spend an absolute fortune on equipment, flavourings, bait and the latest electronic gismo at the end of the day it's like Malc says, it's what ever gives you the most confidence, in your price range and what works for you.
Having seen the advice shared by the wealth of experience of the people that use this site, I am confident that people like Malc could catch fish using a bent pin and a bamboo cane.
My belief is that as with any sport, if you have the best gear it will probably raise your game by maybe 10%. It doesn't make up for experience gained by taking the time to watch, absorb and experiment.
My point really is this, while we are trying all these different baits and flavourings etc, the most important thing is we are gaining experience.
How else can bait like Bread and Marmite be as good as Custard Paste?