I am gonna install Lord Vishnu's deity. So I would like to know what is Lord Vishnu's favourite food.
There is nothing like not favourite item to the Lord. Every item is a favourite to Him. The most important thing the Lord sees in us is devotion while accepting an offering. In Bhagavad Gita, He says this explicitly.
पत्रं पुष्पं फलं तोयं यो मे भक्त्या प्रयच्छति |
तद्-अहं भक्त्य्-उपहृतम्-अश्नामि प्रयतात्मनः || ९.२६ ||
patraṃ puṣpaṃ phalaṃ toyaṃ yo me bhaktyā prayacchati |
tad-ahaṃ bhakty-upahṛtam-aśnāmi prayatātmanaḥ || 9.26 ||
Meaning: Whoever offers to Me with devotion a leaf, a flower, a fruit or some water, I accept this offering made with devotion by one who is pure of heart.
यत्करोषि यदश्नासि यज्जुहोषि ददासि यत् |
यत्तपस्यसि कौन्तेय तत्कुरुष्व मदर्पणम् || 27||
yat-karoṣi yad-aśnāsi yaj-juhoṣi dadāsi yat |
yat-tapasyasi kaunteya tat-kuruṣva mad-arpaṇam || 27 ||
Meaning: Whatever you do, whatever you eat, whatever you offer in sacrifice, whatever you give away, whatever austerity you practice, O Kaunteya, do that as an offering to Me.
So, you can offer anything to Lord Vishnu with a pure heart and devotion. What ever you do, what ever you eat, don't forget to offer it to the Lord.
But if you are very much interested in the food item which is favourite to Lord Vishnu, there is a big list for this. Tirumala Tirupati Devasthanam is one of the world famous temples in the world. The main deity here is Lord Venkateshwara. Many items are offered to the Lord here.
Quoting from this answer of mine,
Lord Venkateshwara is known as naivedya priya meaning who is fond of offerings. Actually, Laddu is not the only prasadam at Tirumala. Besides this many prasadam like different types of rice offerings like pulihora, curd rice, different types of pongali, ravva kesari, butter, curd etc., are offered. In addition to these prasadams, kadambam, dosa made with black gram, ksheerannam, vada, rice made with pepper etc are offered to the lord at different timings starting from suprabhata seva performed early in the morning..... Later many prasadams were introduced at lord's temple like appam, manoharapadi, atirasam, sukheeyam etc.
This is from Tirumala Tirupati devasthanam. In Puri Jagannatha temple wide variety of prasadams are offered. This means they are also favourite items of the Lord.
So, there is a huge list of food items you can choose from. But don't forget... offering with devotion is important.
Lord Vishnu loves food items that are yellow in colour.
Yellow lentils,jaggery, yellow ladoo are considered the favourite foods of Lord Vishnu. Lord Vishnu likes amla gooseberries too.
Devotees offer him yellow lentils mixed with jaggery in order to please him. Even yellow laddoos are commonly offered to Vishnu.