One more vote for KRU/FOO/FOX core with Kylo/FOSFTP as 4th and FOST as 5th being the best pure FO team. Zeta order as above, KRU lead -> FOST Unique -> KRU unique and I’ll add to the above that KRU unique is super useful against JTR teams too since it changes her opening move.
My vote is strongly for Kylo over FOSFTP personally. I like the TM gain from lashout every turn and his stacking speed means he can reach a stupidly insane turn frequency during a matchup vs a debuff heavy team. I only use him in the team for TM gain purpose although his AOE can do respectable damage with good offense secondaries and a CD triangle. Others have had success with FOSFTP though so I'm not discounting that.
There’s occasional (slightly heretical) talk about KRU/FOX/KYLO/FOST/FOSFTP being best on defence given how abysmally the AI handles FOO (often cleansing Kylo’s DOTs or opening with pinning shot etc. etc.). This might be true but FO is still pretty bad on defence either way so probably best to focus on conventional wisdom and keep FOO in there if you are only interested in developing 5 characters for the team.
The thing about FO is beyond the core of the team there’s a lot of flexibility in what is “best” depending on your opponent. Prime example is having Phasma as the 5th (KRU/FOO/FOX/Kylo/Phasma) which is by a long way the most reliable team against Traya lead triumvirate but is not viable against many other teams (or on defence). Again, switching FOO out for FOSFTP (KRU/FOX/KYLO/FOST/FOSFTP), so you have 2 AOE attacks and are a bit tankier, is the more reliable against good NS teams, but FO normally handles NS without any problems regardless.
Extending your original question to hybrid teams, switching out the 5th spot for Thrawn is awesome on offense (KRU,FOO,FOX,Kylo,Thrawn) and even allows you to beat Revan when your FOO has the mod advantage (requires FOO going first, i.e. 13 more speed from mods on your FOO verses what your opponent adds with mods to their Revan in a lineup with both Basti and Jolie). Thrawn also helps vs Palp lead teams but for those teams my favourite to use is Nest for her AOE daze. I also found Nest was the best all round addition in the 5th spot for leaving on defence.
Modding on an FO team is pretty crucial but there are some simple points. The most important thing is getting FOO fast. Then focus on CD primarily and Health secondarily on FOX (but before 6 dot mods you may need a protection primary on the circle for survivability). Next focus should be speed on Kylo (in my opinion) followed by a good balance of defence and health sets with protection primaries (at 5 dot) on KRU. When shifting to 6 dot mods I’d suggest to focus first on health primaries for FOX followed by one or two health primaries for KRU.
Since 13 more speed from mods than Revan is not realistic at the top of a shard, Revan has finally made me switch away from FO which has made me very sad as they are an amazingly fun team to play! So if you are going with them (knowing as I did they don’t hold well on defence but can beat any equally modded/geared team up until Revan) then good luck and have fun!