Which modern IE language is most conservative in noun inflection and in this aspect is most similar language to PIE?
A simply approach to the question is to find which language (if any one language can be determined) has the most similar noun paradigm to PIE. The phonology of the suffixes can also be concidered.
The PIE noun paradigm inflects nouns based on case, number, and gender.
Case: There is some disagreement over how many noun cases exactly PIE had, but it probably had 8. (nominative, vocative, accusative, genitive, dative, ablative, locative and instrumental [and maybe allative seen only in hittite]) In the plural nom/voc are identical, as are dat/abl resulting in 6 cases. In the dual it is simplified into just 4 cases.
Gender was once thought to be Masculine, Feminine, and Neuter as in many ancient (and modern) IE languages, but was most likely simply Animate and Inanimate with animate later splitting into masc. and fem. (no modern IE language has this feature, so I'd say the more recent M.F.N. split is the closest we can get.)
Number was singular, dual (exactly two), and plural.
These nouns were also grouped into 5 declensions based on the vowel stem.
Here is a comparison of a few candidates:
English: Cases: 0 (3 [nom. acc. gen.] in pronouns e.g. he/him/his), Gender: 0 (3 m. f. n. in pronouns i.e. he/she/it), Number: 2 (s. pl.)
Farsi: Cases: 2 (nom. acc.), Gender: 0, Number: 2 (s. pl.) plural only in definite.
Hindustani: Cases: 3 (direct, oblique, vocative), Gender: 2 (m. f.), Number: 2 (s. pl.)
Icelandic Cases: 4 (nom. acc. gen. dat.), Gender: 3 (m. f. n.), Number: 2 (s. pl.) with very few remnants of the dual from Old Norse
Greek Cases: 4 (nom. voc. acc. gen.), Gender: 3 (m. f. n.), Number: 2 (s. pl.) with evidence of the dual in Mycenean Greek.
Getting closer, but still no good match.
- Cases: 6 (nom. acc. gen. dat. instrumental, prepositional) instrumental performs the function of ablative and prepositional the locative Old Russian had vocative also.
- Gender: 3 (m. f. n.)
- Number: 2 (s. pl.) Old Russian had dual, but it is only found in very few situations, where it is reanalyzed as a genitive suffix.
- Declension: 3
- Cases: 6 (nom. acc. gen. dat. loc. instr.)
- Gender: 3 (m. f. n.) masculine is subdivided into animate and inanimate.
- Number: 3 singular, dual, and plural. (s. d. pl.)
- Declension: 10 (7 regular)
- Cases: 7 (nom. voc. acc. gen. dat. loc. ins.)
- Gender: 2 (m. f.)
- Number: 2 (s. pl.) (Old Latvian had the dual. some dialects retain it for certain body parts)
- Declension: 6 (3 m. and 3 f.)
- Cases: 7 (nom. voc. acc. gen. dat. loc. ins.) another, allative is found only in adverbs and is an innovation since PIE. genitive performs the function of the ablative in other languages
- Gender: 2 (m. f.) There is no neuter gender and no animate and inanimate nouns in Lithuanian.
- Number: 2 (s. pl.) (It has also a dual, which is almost unused, except a few words that retain their dual forms)
- Declension: 5 (of which numbers IV and V are very rare)
From this (and other languages I checked out) I would conclude that the modern language closest in noun inflection to PIE is either Slovenian or Lithuanian/Latvian.
- Lithuanian has the closest noun case system its 7 cases almost perfectly matching the 8 of PIE with the ablative lost. But vocative is still in use
- Lithuanian has only 2 genders (m. f.) while Slovenian has the three (m. f. n.) while PIE had 2, they were animate and inanimate, not masculine and feminine. So arguably Slovenian represents the older form which is closer to PIE, the extant neuter being the analog to the inanimate
- Lithuanian has only singular and plural, losing the dual and the complications that come with it.
- Lithuanian 5 declensions match the 5 in PIE
- Slovenian has 6 cases, loss of the vocative and ablative mirrors the PIE plural where nom/voc and dat/abl are identical.
- Slovene subdivision of the masculine into animate and inanimate complicates things. because it is an innovation not related to the PIE genders.
- Slovene is one of the only IE languages that still has the dual in frequent use as part of its noun paradigm
- Slovene has 10 declensions, not quite as close as Lithuanian
Lithuanian seems to match more closely, despite the lack of the dual. That and its declension system, almost identical cases, extant vocative, use of the dual as recently as the 19th century seem to trump Slovene's more closely matching gender and number paradigms.