Personally I'd recommend playing most games without mods first. Specifically to this franchise, I'd even suggst playing Fallout 3 and New Vegas without mods first. There's multiple reasons for this...
1. Experiencing the game "as the developers intended" is a good thing your first time around.
2. Spending time hunting down and compiling mods before you've ever even played the game is silly; what if you don't end up liking it? What if you spend so much time getting mods that it lessens your enjoyment of the game?
3. You don't want to have a bunch of spoilers from reading through Changelogs of mods if you've never played the game. (some would argue this is the most important reason)
4. Mods can and do sometimes introduce crashes; nobody wants to play a crashy game their first playthrough. This is especially true with games like Oblivion and Fallout 3 because there's the tendency to install dozens of mods, which are likely to conflict.
5. You'll appreciate the mods more once you've seen the game, because you'll actually see the difference of what was added and improved.
6. It helps with bug reports to modders once you play with the mods, because you can say "This didn't happen in the normal game"
7. Reading changelogs will likely have spoliers and that's no fun your first time through a game.
The only exception would be pure patches -- Unofficial Fallout 2 Patch, Unofficial Oblivion patch, etc. Things that just fix the game, don't change it. But again don't read the changelogs because MEGASPOILERS OF DOOM