Salutation should be responded in like or with something better. Allaah says,
وَإِذَا حُيِّيتُم بِتَحِيَّةٖ فَحَيُّواْ بِأَحۡسَنَ مِنۡهَآ أَوۡ رُدُّوهَآۗ
‘And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it [in a like manner].’ (Al-Nisaa’, 86)
At least the word Salaam should be there. Therefore, it is necessary when someone says Salam alaimkum wa rahmatullaah wa barakaatuhoo to respond with the same. And if a person were to say Salam alaimkum only then you should say alaimkum salaam. But if you were to add wa rahmatullaah it is better, and if you were to add wa barakaatuhoo it is even better.
There are some scholars who are of the view that if a person responds to salutations by saying ‘welcome’ then that is sufficient. This is because it is recorded that the Prophet ﷺ responded to the salutations of Umm Haany’ (may Allaah be pleased with her) saying ‘welcome to Umm Haaye’’ (Bukhaary 3623). He also responded to the greetings of Faatimah (may Allaah’s Pleasure be upon her) saying. ‘Welcome to my daughter’. (Bukhaary 3623). Hence, they say it is sufficient to say welcome. However, the verse is explicit and other hadeeth indicate that responding to salutations must contain the word Salaam. The response to the two aforementioned hadeeth which suggests that the Prophet ﷺ responded to the greetings of Umm Haany’ and Faatimah (may Allaah’s Pleasure be upon both) is that salutations with Salaam may have taken place but the narrator did not narrate it this is because the narrator saw it sufficient to use the aforementioned word to indicate response to salutations. It is not necessary to transmit the response of salutations in every circumstance. For instance, Ibraaheem (peace be upon him) when the angels greeted him with salutations he responded to them with salutations,
إِذۡ دَخَلُواْ عَلَيۡهِ فَقَالُواْ سَلَٰمٗاۖ قَالَ سَلَٰمٞ
‘When they entered upon him and said, "[We greet you with] peace." He answered, [And upon you] peace’ (Al-Thaariyaat, 25)
Whereas in soorah Al-Hijr is says,
إِذۡ دَخَلُواْ عَلَيۡهِ فَقَالُواْ سَلَٰمٗا
When they entered upon him and said, "Peace." (Al-Hijr 52)
In this instance his response to the greetings is not recorded. This does not mean that Ibraaheem did not respond to the salutations. This is because his salutations have been mentioned in another place. Therefore, if the ruling had been mentioned in one place it does not mean that it has to be mentioned in every place and in every circumstances.