Have you heard of the Arduino platform? It turns out that's really just an ATMEGA328P.
Not only that, but it also turns out that you can now use the Arduino software environment to program ATTINYs as well!
There are a whole slew of different Atmel chips available. As with any sort of ICs, it helps to pick a small subset of chips to "standardize" on, so that you can keep a small stock of chips on hand which are universal enough that you can make a rough proof of concept immediately, and then shop for higher performance exotic chips to improve the design after you get the prototype working.
Personally, I have standardized on 3 Atmel chips: their best 28 pin DIP, their best 14 pin DIP, and their best 8 pin DIP (the ATMEGA328P-PU, ATTINY84A-PU, and ATTINY85-20PU).