Vuman Cavalier Fighter [sentinel feat]. Use a halberd for reach.
- From 1st level you have Sentinel (AoO if your target moves away (reducing their speed to 0) or attacks someone other than you).
- From 3rd level you can 'mark' creatures you attack; they have disadvantage to attack anyone other than you, and if they damage anyone other than you, you get a free attack against them the following round (at advantage, with a slight damage buff, as a bonus action, a few times per day).
- At 4th level, take PAM as your feat - you now also gain an AoO when creatures enter your reach.
- At 7th level, you get a parry ability to protect yourself or adjacent creatures.
- At 10th level, any movement in your reach provokes AoO (and reduces speed to 0).
- At 18th level you get an AoO every single turn, allowing an AoO against every single creature that tries to move away from you, enter your reach, or attack anyone else but you.
For feats other than PAM and Sentinel the usual suspects look good. GWM for damage. HAM gives some nice DR. Martial Adept is useful for a Trip attack against a creature entering your reach (reduce speed to 0, trip them, they cant attack you, and then wail on them on your turn). Lucky and Resilient are great as always. Mounted Combat is fluffy but campaign dependent.
Otherwise boost Con, then Strength (and keep your eyes open for a Belt of Giant Strength).
If your DM uses UA and is a fool, take the Tunnel Fighter fighting style. Otherwise Defense style is good.
For MC options, give yourself a Charisma of at least 13 at character creation, and take 3 levels (or more) in Crown Paladin. You gain a 1/ Short rest lock down of all creatures within 30' (they cant move away from you) with your Divine Channel, and access to
Compelled Duel as a spell (plus a few Smites).
Also very fluffy.