Things that make you OP;
-Upgraded equipment (OP since +8 onward)
-Cash Pets (mainly for +3300 max HP)
-Tons of HP (no doubt, Defender wins here)
-Cash bike (no usage animation)
-Spider mech, but last 7 days
-Medic with full healing skill (heals depend on percentage of target HP)
It's pretty balanced if you remove the Spider Mecha and the pets from the list.
I can wiN against most classes with a Sentinel even tho i have the lowest HP.
Punisher - so is the guy who see's the other one first that wins.
Both are very even, almos the same DPS, Punisher has 2k more HP but Sentinel has the best Survival abilities which lasts for a short moment so the Sentinel must kill the Punisher fast as h e l l.
Shadow - Full spec'ed stealth detection is to easy against them, i can see them before they are even close enough to stab me, they can only win against me if an not watching my back.
If the Shadow gets the first hit so will he always win, high dps and stunlock.
Whipper - Not a problem realy, they are only OP in large PvP like the escort when they can damage 50 people at the same time.
But i have never had a problem against, i can just slow/stun them and kite them around.
Medic - Must have the crit buff and both stuns ready to kill a Medic or they will heal more then you can damage him.
Defender - Nothing you can do against, they have wayyyy to high HP so a sentinel kill them and don't underestimate them, they do a lot more damage then you think.
4k HP vs 21k hp