1998-04-30 original release as reported by Epic Games, published by GT Interactive. Validity uncertain despite the source.
1998-05-22 as reported on the official site at the time of release.
1999 Unreal Gold - re-release that bundled Return to Na Pali.
2000-09-22 - re-release by Infogrames
2001 another re-release by Infogrames
About Unreal
The official Unreal website states that Unreal was released 1998-05-22, and that seems to be the most reliable source to me.
Ah, you're correct. Odd the developer states it to be end of April in more recent information sources for it.
Edit: Restored the edit you made, sorry about that. Also edited the OP to reflect this.
Edit: Restored the edit you made, sorry about that. Also edited the OP to reflect this.