For me it has to be Gnarly's Tale of Two Wastelands repack. For those that don't know Tale of Two Wastelands combines the two games allowing you to travel between the two through a metro line. Before Bethesda removed the dated GFWL DRM from Fallout 3 (on Steam) it's what a lot of people recommended instead of GWFL remover mods. Even though I own multiple copies of both Fallout 3 and NV on multiple platforms it's significantly easier and faster to use the repack. Probably saving hours of time especially if you are attempting to play the game on the Steam Deck.

Some follow ups would be FitGirl's Sims 3 repack which contains the Overwatch & ErrorTrap mods and FPS Limiter/3Booter utilities and MagiPack's Sims 2 repack that contains the Sims2RPC launcher and 4GB patch. Both contain mods that improve the games' performance and stability.

I would like to encourage people to check out MagiPack's repacks by the way. They do some fantastic work and specialize in making sure older games (largely pre-2009) work on newer OSs but they aren't really discussed in a lot circles.