Hard not to pick the Holorifle, but I've always liked the All-American. Once I found out how quick it could fire (as long as my finger could go fast enough), it became a formidable weapon. There are other rifles that I really like, but I much prefer a scope. If the All-American was silenced, it would rival my favorite from Fallout 3: the Perforator.
Weapon Mods Expanded
(from readme file)
Marksman Carbine / All-American
+ 5.56mm Carbine Extended Mags - Increases ammunition capacity (+6).
+ Marksman Carbine Improved Rifling - Increases damage (+12.5%).
+ Carbine Suppressor - Silences the weapon.
found here: http://www.newvegasnexus.com/downloads/file.php?id=39651
Downloading and using this mod refreshed a stale game, for me*, and made a lot of the weapons much more fun to use- yet it doesn't make any of them game-breakingly overpowered. Highly recommended.
* - along with Project Nevada, Fellout (much-needed world graphics improvement), centered 3rd-person view, and several other minor-but-nice problem-fix mods.