Sentinels are great harrassers, obviously, with their scouts rule.
with the new Astropath mix, a unit of those with heavy flamers can do ungawdly things to soft squishies in cover (outflank and dig out pathfinders/grots holding objectives/cover hiding lootas etc etc).
Knowing who is going first with a scout move can allow you to move em out (if you don't outflank) and know they'll be in range of a front rank/infiltrating unit after your movement (barring 'seize the initiative').
Plasma cannon/hvy sentinels...armour is nice, but I think you are better served with the outflank/maneuverability of the original scouting version.
I do NOT like rough riders beyond fluff...they are very one shot-ish...but if you are using Ursakar Creed, they are one of the units you can assign scouts to for a nice nasty outflanking death charge...just accept they will die immediately after.
"You've heard of animals chewing off a leg to escape a trap? There's an animal kind of trick. A human would remain in the trap, endure the pain, feigning death that he might kill the trapper and remove a threat to his kind."
"The best years are the forties; after fifty a man begins to deteriorate, but in the forties he is at the maximum of his villainy."