Well, I often delay evolution of pokémon whose design I really like, unless they learn a really good move it evolved at the earliest level. And example of the former would be Quilava, I delay its evolution until before Clair's gym if using it on HeartGold. Mudkip on the other hand, has to be evolved at lv.16 otherwise it's actually harder to use than the other starters - its type advantage over Wattson being completely wasted.
Poliwhirl is another I often delay, as I like the design of all 4 members of that family. And I also like utilising its speed for a bit before evolving into a slower pokémon.
Pikachu is one I've never evolved whilst using it as a team member. Mainly as I've only used it once, on LeafGreen when doing a game using PokéSpe Red's team. I evolved it once I was ready to move it over to DPtHG though.