1.7.10 (or 1.6.4, or 1.5.2 for that matter)

1.3 was another nasty one. That was the one where multiplayer and singleplayer were merged and LAN play was introduced. Before that mods were released specifically for either single or multiplayer and authors would have to specifically build 2 versions of every mod if they wanted to enable use in multiplayer. This shift killed a ton of mods and version 1.2.5 was a peak for mods like Red Power for example

If you use an alternate launcher like Prism Launcher it is trivial to install tons of modpacks for any version of Minecraft and manage many different mod loadouts (with handy search and auto-download of both modpacks and individual mods, plus it makes it super easy to modify a modpack you downloaded and add/remove mods) Its really the best way to play modded Minecraft (and has been since the fork from MultiMC) plus unlike most launchers which are super-simplified to not scare newbies, Prism Launcher also exposes tons of handy technical stuff if you want to dive deeper, such as optionally displaying full logs, java version and argument management, world edit and other tool integrations and more.