It really depends on what you want t do. When you evolve something, the level and IVs remain the same, but the Pokemon's base stats change (almost always going up) and the moves will get re-rolled (and sometimes the evolved Pokemon has different move possibilities). Because the base stats go up, evolving will pretty much always make the Pokemon stronger.
If you want the strongest Pokemon possible eventually, then IVs are the main concern. You can always raise their level later (by powering them up via stardust investment). If you want strong Pokemon now and don't plan to power them up, then IVs don't really matter and you can just evolve your highest CP one.
In practice, it's probably best to go for a mix depending on what Pokemon are common in your region and which ones you want to use. Aside from all the mass evolution fodder, I'm mostly reserving evolution of uncommon/rare and powerful Pokemon for those with decent IVs, which usually means hatched Pokemon. For common ones that are decently strong, I'm still waiting for high IVs but I'll also use excess candy to evolve some high CP ones too. For example, I have tons of Nidoran (both male and female), Clefairy and Drowzee candy, so I've evolved a few of each based solely on CP. I'm not going to power them up, but they're useful for attacking and holding gyms now and again. I still have plenty of candy remaining in case I ever get a great specimen.
These aren't strict rules, of course. I've evolved some Pokemon with 80ish IVs rather than waiting for 90+. My Victreebel is 87% but I've invested some stardust into it now because it has the best moveset and evolving another one is an unnecessary gamble.
tl;dr - it's ideal to get one with high IVs AND high level, but it's not really practical because it's tough to find both.