OP, this is too general a topic to answer. But some guidelines:
1) Breakfast isn't usually complimentary. The exceptions are certain locations (e.g. Scandinavia), lower-end chain hotels (e.g. Holiday Inn Express), and small independent hotels (often family-owned and operated). Or you hold elite status with the chains.
2) Breakfast at chain hotels are very predictable and you know what you are getting at each star level. At the top end you can expect multiple buffet stations (hot and cold) and a la carte ordering (e.g. egg white benedict or fresh-made pancakes/waffles), fresh fruit and veggie juices, and other more exotic items (e.g. real honeycomb). The quality is very high and you seriously won't need another meal until late afternoon. Price usually around 30EUR per person.
3) Quality at small independent hotels vary greatly. Check reviews. But in general don't expect too much, and it's certainly nothing to wake up for.
4) Location of the hotel. If there are plenty of options outside the door, breakfast isn't such a big deal. However, if the next place where you can buy/eat something is 20 min walk away, go figure.
The most important criterion to consider is actually the purpose of your trip and your schedule. If I'm partying every night, I'd almost feel better not to have breakfast comp'ed, or it's a dilemma. On the other hand, there have been work-heavy trips where I literally holed up in the hotel and living from comp breakfast and lounge snacks