I prefer to use pokemon that i like however i've restarted so many files on various games that i have been using pokemon i have not used before and not necessarily pokemon i've always liked. Not necessarily pokemon that are considered strong either.
Machamp and Staraptor for example are both pokemon i've always thought of as mediocre and not very interesting but in my Platinum file i used them as i had never before and they are quite good. I still do not favour them but i like to vary my teams as best i can throught my games.
Primeape and Dugtrio are also pokemon i have never really liked atall but in two different LG files i used them. However the Dugtrio was soon replaced by Rhyhorn as i prefer it and Rhydon to Dugtrio. Plus Dugtrio had horrible defence, rockets at Silph were fainting it after two or three hits even though it was about 5 or 6 levels higher than their pokemon. So that was a mixture of preferance and strength.
On my current LG file i have a Dragonite, considered strong and i like it, but oddly have never ever used one even back in the R/B/Y days. I also pick pokemon according to how they will balance my team.
I have yet to challenge the battle tower on any of my games and i am considering using strong pokemon for that, however i will still not use pokemon that i do not like.
I one day plan to get a Pearl game or restart my diamond but even though i have chose Turtwig and Piplup in the past i will never choose Chimchar because i do not like it and it's evolutions atall even though i know they are good pokemon.
My shiny team is full of pokemon that i like, Slowbro is part of it and i have never really favoured it but i really like it now. She was originally chosen to balance up my team as it lacked a water type and i had other options such as Floatzel, Pelipper ect however i preferred Slowbro.
Pokemon i will never use in battle because i do not like them include, Spinda, Volbeat, Illumise, Roselia/rade, Gardevoir, Shiftry, Slaking, Lickilicky, Magmortar, Probopass, Mr. Mime, Jynx, Delcatty, Purugly, Skuntank, Hariyama, Ambipom.
Saying all of this there are still pokemon that i have never used but i like very much. These include Skarmory, Lapras, Nidoking, Vileplume, Aerodactyl, Ariados, Heracross, Scizor, Donphan, Rayquaza, Tangrowth.