I don’t use much. Something to boost the signal for leads, which can also be just the lead channel of the amp, and I like to use a slow delay with a treble cut-off for melodies and slow solos. I don’t have any favorite brands yet. An octaver (down) and a wah-wah can be fun toys, but nothing essential.
The absolute best atmosphere for song writing is found in solitude late at night. Preferably after midnight. The main reason for that is the silence and the very few interruptions. No telephones that need to be answered and no sudden important situations that need to be dealt with. This, together with some black coffee, offers a nice harmonious sanctuary ideal for creativity.