With insect repellents, the scientific evidence says that DEET is the most effective, and it is in many repellents, such as Life Venture, Jungle Formula, Boots Own Brand (in the UK) etc. Some people don't like using it because of the solvent in it.

Natural formula repellents containing PMD (Citriodiol or Citrepel) such as Mosi Guard and Boots Own Brand Natural Repellent, Dr D and Dr Johson's (both available in Kenya) are also effective but may need re-application as the effect might not last as long as DEET based ones. Icaridin (sold as Picaridin) is relatively new and is reported to be as effective as DEET but without the nasty aspects, as with PMD it needs more re-application than DEET.

The percentage of DEET refers to how long the application lasts, not how effective it is at keeping insects away, see this from Guidelines for Malaria Prevention in Travellers from the UK:

"Duration of protection is 1 to 3 hours for 20%, up to 6 hours for 30% and up to 12 hours for 50% DEET. There is no further increase in duration of protection beyond a concentration of 50%. Sweat-off time varies with activity. The interval between applications depends on this as well as the DEET formulation and concentration used."

Be careful with it, as well as removing nail varnish:

"DEET applications can damage some plastic watch straps, watch glasses and plastic jewellery; these items should not be allowed to come into contact with DEET."

There is no advantage to using anything over 50% DEET which is only used on mosquito nets, clothing etc.