For me I usually just choose in game pokemon mostly because I like their design, or I've never used them in a team before. Sometimes though the competitive battler in me cringes when I go to choose something most people would class as 'weak' or 'useless.' Right now, I'm considering a jumpluff for my SS team just because I like the design and it has a decent support movepool; though the meager stats are putting me off (any advice there?)
So which do you go for? A mixture of both? What classes as a 'strong' pokemon for you when it comes to in game teams?