Janemba is a character that's built around his teleports and neutral tools. Together they allow him to have fearsome left-right mixups and an iron grip on neutral. 236X deflects Ki Blasts and his own Ki Blasts track, allowing him to catch the opponent anywhere and stopping them from firing back. On top of those tools, his 22S completely shuts down projectiles and assists, letting him run in for free pressure or even a combo.
Janemba's worst position. Some of the biggest strengths of the character are his A assist and level 3 mix, neither of which you get easy access to on point. Janemba's poor roundstarts and the fact that he doesn't really get as much out of assists as most characters are just more reasons to not run him point.
Arguably his best position. At mid, you can use his assists for your point while leaving an assist for Janemba to help with neutral and mix. Additionally coming in with meter from your point lets you take advantage of his level 3 mix, and Janemba still does enough damage to two touch characters reliably, even without limit break.
Janemba is a good anchor. He does ridiculous damage with limit break, solo 50/50s courtesy of level 3 and generally doesn't need assists to function. Anchor Janemba does have some issues, such as struggling to get in at times, but the aforementioned strong points combined with his ability to stop assist and projectile spam make him a very capable anchor character.
Ultimately, Mid and Anchor are his best positions. Which position he's put in really depends on your team and how comfortable you feel with solo Janemba.
Janemba isn't super picky about his teammates, and most of the synergy comes from how well your other characters can use his A assist, since B and C are fairly standard.
In general, characters that get a lot out of Janemba A are ones who:
Have high hitstun, long range tools
- SSGSS Vegeta, Jiren, UI, etc.
Enjoy being able to end combos with air DR
- Vegito, Broly, A21, Teen Gohan, etc.
Don't really need blockstun assists for their gameplan
- Cooler, Jiren, Vegito, etc.
Have tools to beat superdash (air dps, large air normals, etc.) and good anti-reflect strings
Janemba A is also one of the best assists for Nappa and Majin Buu's restands and can give character specific stuff, like letting Base Goku charge Spirit Bomb midcombo.
Most Janemba synergy is fairly one sided, but here are some things you should look for when picking his teammates:
- Cooler A, Hit A, Roshi B, Nappa B, etc.
Certain high blockstun assists give him a 50/50 midscreen
- Bardock B, Jiren A, etc.
Characters that can reliably dhc and properly space Janemba's level 3 for his 50/50
- The pinned doc in the discord goes into more detail, but most characters can do this in some way
Strong neutral assists that Janemba can easily convert off of thanks to ki blasts and j2M
- Beams, Krillin A, etc.
High meter build and corner carry let you go into level 3 mix as frequently as possible
- Jiren and Teen Gohan are particularly good examples
As long as a character can reliably do at least two of these for Janemba, they should make a good partner for him