Using the 50% uptime assumption for sentinel. I can see a ranger 5 / rogue 1 with sentinel being a beast.
1d8(rapier)+7(stat and duelist)+1d8(colossus slayer)+1d6(hunters mark) + 1d6(sneak attack)= 23 damage. 15 dmg (if colossus slayer has sneak attack both cannot be used).
That's about 776 damage per day. (assuming a 65% chance to hit).
One minor damage difference is that if you are 5/1 you only have, at most, one ASI so your stat bonus will be +4 if you are variant human and +3 if you aren't (and you'll have a lower hit chance, too)
A slightly different take is instead of Sentinel and Colossus you take Shield Master and Horde Breaker, though that requires a high Strength to work best. You can use Shield Master to either knock prone and get advantage for you and your allies or move 5' for a chance to use Horde Breaker.
There are multiple possibilities here. You knock someone prone AND he's 5' from a foe. You DON'T knock someone prone AND he's 5' from a foe. You knock a foe prone AND he's NOT 5' from a foe. You DON'T knock someone prone AND he's NOT 5' from a foe.
All the above cases assume you can get a sneak attack on every attack because of an ally within 5'. I won't look at the possibly (yet) of what happens if you don't have an ally within 5'.
The first case is the best case scenario and it's a bit of a mess with two different hit probabilities and three opportunities to land one sneak attack. Also, no Hunter's Mark as your Bonus Action is also using your feat. Also, variant human so your Strength is 18 at by level 6.
First (and best)case scenario: 25.4 DPR
Second case: 20.9 DPR
Third case: 19.4 DPR
Fourth case: 14.8 DPR
Over 24 rounds that's somewhere between 355 and 610 damage at level 6.
It's not particularly amazing but it does get (a lot) better as you get more sneak attack dice, you don't have to use Relentless Attack and open yourself up to counter attacks, the Shield Master 5' shove is incredibly valuable above just defaulting to knocking someone prone, you can get that second attack at level 3 Ranger instead of waiting until 5, you can actually look at doing something else with your Ranger spells besides cast Hunter's Mark, and being a variant Human Ranger/Rogue you'll have a jillion skills.
Major downside, you've probably severely reduced your use of Cunning Action, one of the games best abilities. Though it does work with any Rogue subclass so you can go Swashbuckler and you can still escape or Sneak Attack more often.
It's exceptionally versatile. Heck since you probably have a 14 Dexterity, you can still shoot a bow 2 or 3 times and Dash or Hide if you are miles away from the enemy.
