When looking for teammates for Sentinel you either want an assist with good horizontal coverage(like drones, which unfortunately sentinel can't take advantage of) or one that makes it easier for him to control the air like hidden missiles. But the cool thing about sent is since he is such a versatile character, he can use a variety of assists. The only assists I would shy away from are lockdown assists not named tatsu, sentinels only real grounded mix-up is cr.M or command grab, a mix-up that is completely nullified by an assist that keeps the opponent in block stun for an extended period of time. I made the exception with tatsu because its extremely fast(unlike say coldstar) and can be used for fly cancel hit confirms(off both a cr.M and a j.H). But even with coldstar, Im sure if you so wished you could find a use for it, my experience with it wasn't pleasant but that could just be due to my incompetence.
The top assists for sentinel in no particular order would probably be:
Hidden Missiles, Akuma Tatsu, Any horizontal projectile(plasma beam, disruptor, greyhound, unibeam etc), Jam Session, Peekabo, Vajra.
As for team order placement, personally I feel sentinel is best played when he is 2nd. Playing him on point is workable since he has 2 assists behind him, but your giving up access to one of the top 3 assists in the game, so unless those two people behind him are doom and akuma, your scarcely gonna find another character in the game with a better assist. Plus you also sacrifice access to his really deadly cross over counter into hard drive that has killed many an over-zealous character and assist. But why not play him last you ask since most characters benefit more from his assist than he does from theirs, well because sentinel on anchor-while not nearly as bad as everyone would like to imply he is-is very hard to make work. Plus he has bad match-ups against most of the top tiers so if your ever in a situation where its one on one(which tends to happen quite frequently in this game) you'll more often than not find yourself in an uphill battle.
The 2nd slot on the other hand is perfect for sentinel because its the best of both worlds, you still have access to his amazing assist, but sentinel isn't gonna be forced to fight alone. Plus with my aforementioned point of sent being able to escape a lot of incoming character mix-ups, he can come in and slow the game down if it looks like your about to be steam rolled. Finally, your point character is given access to the plasma storm dhc, which is one of the most damaging dhc's in the entire game. Plus, depending on who your anchor is, sentinel gets the ability to combo his H's into HSF from full screen. Ryu, akuma, storm, trish, hawkeye, and dante are a few characters, but anyone with a fast full screen hyper can do this. All in all, Im a strong believer that 2nd place sentinel is the best spot for him.
With that said, if you feel like running him on anchor don't despair, it is possible as rayray and infrit have shown numerous times to win with just sentinel since he has very high damage output, the ability to play either rush down or keep away, nice incoming character mix-up and has really good hypers.