I would start with the first .hack games, g.u. last record or link to get a gist of what the series is about. Each media however is usually both it's own thing but connected in the background. Timeline wise, below is how the series operates.
LN: Epitaph of Twilight
LN: AI Buster 1
LN: AI Buster 2: 2nd Character
Anime: Sign (Episodes 1-26)
Anime: Sign (Intermezzo)
LN: Zero
Game+Anime: Infection > Liminality Ep1 (Mai Minase)
Game+Anime: Mutation > Liminality Ep2 (Yuki Aihara)
Game+Anime: Outbreak > Liminality Ep3 (Kyoko Tohno)
Game+Anime: Quarantine > Liminality Ep4 (Trismegistus)
LN: Another Birth (The games from another perspective)
WN: New Trial (Web novel of games with new background information)
Event: Zero - The Phantom Mind (Original Version of Events General Story shown in Link)
Manga: U.D.D.E.N. Complete Book - Ouka's Story
LN: AI Buster Rumor
LN: AI Buster Kamui
Anime: Sign (Unison) ((PXZ1)) & ((PXZ2))
Archive 03: Legacy
LN: AI Buster Firefly
Manga: Lucky Hack Pack & Rena
Manga: Legend of the Twilight Complete Edition
AD: Let's meet offline
Event: Chap Chop Case (Original Version of Chap Chup Case from Link & No not a typo)
Game Report: End of The World
Event: The 13th Bansyoya File
SS: G.U. Trilogy New World Guide: Shugo & Rena
Manga: Gaspard's Go Go The World
Manga: Alcor
AD: First Login
Game: GU Rebirth (Stop after tutorial quest)
Anime: Roots (Episodes 1-12)
AD: Roots Sound Form 1
Anime: Roots (Episodes 13 - 17)
AD: Roots Sound Form 2
Manga: GnU
Anime: Roots (Episodes 17 - 20)
LN: Cell
SS: Journal of The Dark and The Wicked
Anime: Roots (Episodes 21 -24)
Anime: Roots (Episodes 25 -26)
Game: GU Rebirth (Continued) > Online Jack
Game: GU Reminisce > Online Jack ((PXZ2))
Game: GU Redemption > Online Jack
AD: GU Radio
Manga: 4koma - Peco's Story
Anime: Roots (Returner)
SS: GU After Story
Anime: GU Trilogy (Movie Sum. - Ending Credits)
AD: Innocent Call
Game: GU Reconnection
LN: GU Ragtime
Play: End of The World R:2
Manga: LINK - Twilight Knights Bonus Chapter
Game: Link (Main Story)
Game: Link: (Main Altered Timelines)
Game: Link (Extra Generated Timelines [Main Continuity Versions of Previously Parallel Materials] :
1. GIFT
2. XXXX
3. G.U. +
4. G.U. Novels / GU Epitaph Users
5. G.U. Trilogy )
Game: Link - X'over
Game: Link: Integration Unision
Play: After 2021
Manga: Quantum (I)ntroduction
Anime / Manga / Novel : Quantum, +, Twin Hearts
WN: Bullet
Anime & Manga : Beyond the World
Game: Versus > Thanatos Report
Game / Visual Guide : Guilty Dragon
Game: New World
Aún no. Sin embargo, debido a la cantidad de tramas de fondo que ocurren a lo largo de cada temporada, podemos suponer que algún día habrá más contenido.
Yes. And .hack has multiple avenues to pursue that. Project N.U. is technically on-going. Innocent Call teased more information to be discovered about the New AIDA Council. Fanbook Vol 4 had appearance of an unknown character with a scar and a cigarette (Despite what fan artist try sell you however he's not Kite, We already have an idea how Kite looks later on). Many mysteries still exist regarding Jun Bansyoya. There is era in R:1 involving Project G.U. that can be explored. To this day even though Link covered it Hiroshi still considering creating a .hack//Zero vol 2. Fans continue to demand a remaster, remake or re-release .hack series And to bring it back .hack bullet is soon getting an official digital release along side 43 chapter called versus magician. I could go on and on.