By moving an ally to trigger an opportunity attack, Sentinel allows the user to attack, triggering Extra Attack and Improved Extra Attack so long as the user has not yet used their attack action this turn.
Presuming that Wild Strike and Improved Wild Strike are also triggered by Sentinel, I believe that Bladesinging wizard, Beast Master ranger and Circle of the Moon druid would likely be the best potential abusers of Sentinel in a solo playthrough. Each has the ability to make two or more attacks per round with high-quality summons.
I will discount the wizard for now, as Sentinel seems awkward with their preferred hit-and-run playstyle induced by Bladesong Impeded pushing them towards light armour and cloth.
The druid, presuming that Wild Strike works with Sentinel, may have the strongest late game with Tavern Brawler, Alert, Sentinel and Earth Myrmidon to trigger Sentinel, though between Tavern Brawler and your best summon being Flaming Sphere or scrolls until level 7, this build doesn't seem like it would abuse Sentinel until the mid-game.
The ranger, on its face, looks like it should be the best abuser of Sentinel. Two-weapon fighting gets you four attacks with your full attribute modifier at level 4 and 6 at level 5. Ranger Knight allows you to wear the Adamantine Splint. Hunter's Mark adds to all 7 of your attacks every turn and does not have the harmful flag, so can be used to trigger surprise rounds without costing you an attack.
The problem? Ranger's Companions are not designed to take the brunt of enemy attacks plus opportunity attacks every turn. The wolf, the bear and the boar seem like the most suited to tanking and none of them will last more than a round or two and require significant healing afterwards.
In the late game, one might use Viconia's shield to bolster their companion with Warding Bond, but until then, what? Abyss Beckoners seem ideally suited to this role, but it's unclear how to prevent your companion from going mad. Hellrider's Pride could potentially give resistance a few times per long rest, but as a half-caster your spell slots are limited and a ranger's healing costs an action or at least part of one with thrown potions. A one-level dip into fighter at level 6 could make sense to get the Protector fighting style, meaning that this would be a sword-and-board build from the end of act one.
It all feels very close to working, but I'm struggling with the last few tweaks required to make this a truly great build.
Thoughts and opinions appreciated, particularly if you have tried something similar yourself.