My choice for years was Carnation Breakfast Bars, along with a packet of Carnation Instant Breakfast drink mix (and powdered milk of course) - alternate days were oatmeal.  For years, that was the only choice, but there is a lot more competition now.  Haven't seen the Carnation Bars for a couple of years now, so maybe they don't make them anymore?   If you can find them, they are a good choice.   Just go to any large grocery store and you will most likely find a whole section of this type of product - that's what I do now, and just select by flavor choice rather than brand name - they are mostly all about the same in nutrition.  These type of products aren't cheap though.

I aslo get those Quaker Oatmeal bars? or whatever they are called now.  A good portion size, since they are a solid chunk of oatmeal, more or less - Rasberry Streusel and Apple Cinnamon.  they are supposed to be the equivalent of a bowl of oatmeal, and one thing I like is that they aren't overly sweet.  Another good thing is that they are "solid", and don't get all crushed or crumbled up in a pack.  I'll even occationally bring one to work to have with morning coffee.
I just bought 2 boxes on sale last week, 2 for $5 I think it was, but the regular price is maybe $4 a box, for 8 bars.