Firstly I wouldn’t suggest starting unless you have at least an all blue deck. Otherwise, you will not get very far
My opinions on the various units:
Paladin – Purple especially is strong, blue is okish. Low cooldown so excellent for dealing with fighter/sin rushes especially with multiple paladins/guardians.
Beserker – Little overrated, has some use as a damage soak, but usually it will be dead before it can do any real damage.
Sniper – completely useless, in fact outclassed in every department by the shaman.
Adept – Long turn timer limits its use in pvp although it can be useful behind a warrior. Shaman is a better choice overall.
Guardian – Like the pally, an anti rush unit with the added advantage of 1 extra armour, but without the heal. Again, multiple pallys/guards are best.
Assassin – Rush unit, very good for offense, although its jump can be a bit annoying when trying to defend. Overall very strong, especially the 5 damage version at purple.
Deadeye – Not a fan personally due to the long turn timer – but a better choice than a hunter for a range unit. Could be useful with a tanky setup.
Priest – Long turn timer and low damage means the priest is poor for pvp. pally a much better choice if you must use a healer.
Warrior – Has its uses, especially at purple. Can be vulnerable to assassins and rogues though, and has a very long timer.
Fighter – Rush unit, and very powerful at purple. Use is more limited as the game goes on and there are more units capable of stopping it.
Hunter – Low hp and only one more damage than the deadeye. Only useful behind a tanky line, and even then will go down with just one skill card. Adding the long turn timer, I don’t think this unit is too great.
Seer – High hp anti rush unit with a low timer. Very useful, even more so at purple.
Knight – Slow. not really useful, even at purple, a warrior is a better choice of tank.
Rogue – Good candidate for the strongest overall unit, rogues are very powerful – even the blue is able to 1 hit most units. One of the only really useful units with a long turn timer. Most of the strongest players have a few rogues mixed into their decks
Jester – quite useful behind a line and at the start, but low hp. Jester is probably only worth using at purple.
Shaman – Useful, but the unreliability of the freeze and low hp means there are probably better choices.
Precise Sniper: One of the best skill cards. Works especially well with a rush deck.
Hellfire: Not recommended in pvp, an extra turn to wait and not particularly powerful – most units will be dead long before the burn ends, and rush units can usually be one hit with a snipe.
Punch: Useful for dealing with pesky rogues at blue+. Works well in an antirush deck.