May be the SmArta Sect.
Actually, as far as i know, no sects explicitly say that Manu Smriti, in particular, is one of the fundamental texts for them.
But on the other hand, Manu Smriti is one generally followed and fundamentally important Hindu scripture, which must be important for all sects too.
The Smritis are authority in Hinduism as long as they do not go against the Sruti i.e the Vedas. So, they are authority too, as they are Veda-Mulak (i.e which describes/elaborates the Vedic religion and which has Vedas as it's root), and also because, for the most part, they do not contradict the Vedas.
From MimAngsA ParibhAshA:
- DharmAdharma Nirupanam-- Tayoshcha vedah smritih AchArashcha pramAnam |
Tatra vedah swatantra pramAnam itarau tu vedamulakatayA
Determination of Dharma and Adharma:
[Among those that instruct regarding Dharma-Adharma]- each of the
following is an authority or standard- Vedas, Smritis and ShishtAchAra
(the conducts of the righteous persons)- and among which Veda's
authority is independent. But Smriti and ShishtAchAra are not
independent PramAnas, as they are authoritative, as long as they are
following the Vedas.
- SmrityAdi PrAmanyavichArah-- ManvAdipranitAnAm smritinAmapi vedamulakatvAt swAd ashtakAdau dharme prAmAnyAm||
Investigation of the authority of the Smritis
Due to the fact that the Smritis, composed by Rishis like Manu, are
Veda-mulak, "to-be-dones (Kritya) like Ashtaka etc" are all to be
regarded as authoritative injunctions [although those injunctions are not mentioned explicitly in Sruti]. (MAgha month's
Krishna Ashtami's Pitru shrAddhas are called Ashtaka).
So, since Smritis (like Manu Smriti) are considered as Veda Mulak, a sect which does not follow them, might be regarded as one going against Sruti as well.
And, after Sruti, the Smritis are the most authoritative. So, most sects should be following the primary instructions found in them, at least, besides following the ones which are canonical for them.
So, the conclusion is, except the so-called SmArta sect, no sects to my knowledge, explicitly declare that Manu Smriti is one of the most important texts for them. It is actually not required to do so either, because the instructions given in the Smritis must be followed by all Hindus to a large extent, as not doing so might lead even to Veda Droha (going against the Vedas) for the VedaMulaktva reasoning mentioned above.