The best water conditioners are StressCoat and Cycle. Most conditioners that come with Betta kits are actually barely of any use to improve the water quality for the fish. If you keep your Betta in a filterless environment, I highly recommend the use of StressCoat. If he's kept in a filtered environment, StressCoat in combination with StressZyme are best...

Here are links for the two products:
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=4025&N=2004+112993

http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=4008&N=2004+112990

Any water conditioner will do. Aquaplus, NovAqua+Amquel, StressCoat, Prime, all of these do what is necessary to maintain fish survivability in our chemically treated water: remove chlorines and chloramines, and get rid of heavy metals. A lot of people use StressCoat for the aloe vera in it because they believe it promotes a healthier slime coat. A fish with a good diet and clean water has a healthy slime coat and will not need any extra buffing.

Cycle is not a water conditioner and I do not suggest using it as such. Unless you are planning to cycle your tank, I would not get it.

I agree with minaku. Virtually any aquarium water conditioner will do the job and healthy, properly nourished fish kept in healthy water have all the slime coat they need.

I keep many tanks so buy the concentrated water conditioning products sold for ponds. I just diltue it with distilled water for the tanks and it works just as well as the pricey stuff sold in little bottles.

But not all tap water is equal, so first test yours to make sure what is in it that you actually need to remove.

Cycle is a waste of money, it can actually do more harm then good in some cases, and its expensive. I like Prime because it does just what the other top brands do for a cheaper price. It is highly concentrated and comes in a smaller bottle so it looks like a bad buy. the truth is it costs 3 times less(on average) then Amquel. All the conditioners do is remove chlorine and detoxify ammonia as well as chloramine. It also detoxifies Nitrates, nitrites and some heavy metals.