For best functioning of the circuit, I mean.It depends, amongst other things, on what your definition of 'best' is.
You don't understand that qualitative definitions are useless in this context. What @Alec_t is trying to say, is that you need to come up with quantitative definition of "best". Yes, that means you need some numbers. Coming up with numbers is hard when you are just starting to learn what you are doing, but give it a try because it is the only path to success. It is the only way to measure your progress.For best functioning of the circuit, I mean.
This question is so vague it is impossible to answer. And "best functioning of the circuit" isn't any improvement.Which are the best op-amps models for rectangulare and triangular generators ?
Frequency 800 Hz, amplitude 5-8 V and power supply 15V.What frequencies and amplitude do you want out?
What power supply voltage(s) are available?
In lieu of actual information the default answer is LM358 or TL072 for most of the audio range.
If that is all you need, then pick one that will operate from a single supply, that is also in your price range. The choice is not criticalFrequency 800 Hz, amplitude 5-8 V and power supply 15V.