My opinion is somewhat biased (

) but I like
Fall of the Rising Sun (FotRSU), currently standing at v1.8 plus a small patch. It is a re-build of the old AOTD_MadMax team's Fall of the Rising Sun (FOTRS). We are currently working on the next release. However, there is still "competition" for the player's attention and Bubblehead1980 put together an updated TMO in
Trigger Maru Overhauled Update BH V2.0, where he took TMO and parts of FotRSU, and mixed in some of his own doings for the mod. That mod is also being edited for future updates. Then there is the "reliable", "predictable", "ubiquitous" original TMO
Trigger Maru Overhauled. For that, be sure and download aanker's
1_TMO_25_small_patch to eliminate the Tambor "bug", which isn't really a bug, but a design feature for the player wanting to play the Tambor Class from Start to Finish in the game. The Small Patch eliminates that. You can also sprinkle
Run Silent, Run Deep - The Campaign for even more fun.
There are, of course, other choices, of which my brain is not pulling in any more names... but there is nothing wrong with making multiple copies of your SH4 install, each in their own uniquely named folder, and with the use of
MultiSH4, having unique Save folders for each modded copy of the game. Just be certain that you do not install the game into either of the Windows Program Files folders, else you will be frustrated when attempting to apply mods. Rather, create your own folder, such as "C:\Games..." or whatever, and install into that. The use of the
4Gig Patch or
Large Address Aware (LAA) (that's a write-up from CapnScury) is generally a good idea also. Those utilities are included in the FotRSU download.