Well I've played tons of Gwent, missed 3 cards on the first playthrough so went back and got all the cards on my second playthrough. That being said, I've had a great chance to play all the factions and many different deck set-ups. My favorite is certainly the NR.
For starters I believe they have the best faction perk. Winning a round = +1 card which is very strong considering how a primo strategy is to win by running your opponent out of cards. The Monsters perk lets you keep a card you've already played on the table after each round, but this can be hit or miss whether the card that stay is solid. For maximum strength in a Monster's deck build it's best to include monster cards that have the "muster" special ability which can turn a card with a strength of 4 into a power of 16 with the rally for example. Because of the high level of rally, by nature many of these monster cards must be low (2 - 4 per card on avg for example). As such, there's a high chance that the card that stays behind from your monsters deck is a lone low level 2-4 power card. Whereas with an extra card per round from the Northern Realms perk, with proper deck management you can almost guarantee you will pull a strong utility card or higher power unit card. Meaning with the Monsters perk it's chancy whether the perk plays to your favor, and I'd always prefer something closer to a sure thing (such as with the Northern Realms perk) than to rely on chance to keep my powerful cards on the table.
Secondly, the Nilfgaard perk just seems lackluster all around. Sure it can be useful when playing an opponent who has passed for the round and lets say you can tie the match with a single card and thus win the round due to the faction perk whereas other factions in that situation might have to play a card to tie the score and then another unit card on top to take the lead and win the round, this situation occurs so infrequently that it's much more beneficial to get an extra card each round you win.
Also, the Scoia'tael perk that also you to choose who gets to go first seems completely useless to me. I've yet to find any advantage whatsoever to going either first or second, and I usually don't even pay attention or take-note of who is going first in a match without any adverse affects on my playstyle.
Next, I also think Foltest Lord Commander of the North is hands down the best commander card in the game which is naturally a commander card for the Northern Realms faction. Sure many other leader cards have solid abilities, but many focus on a single row such as doubling the strength of a row of your units or bring a debuff weather effect down upon a single type of enemy units. However, the Foltest Lord Commander of the North ability clears all weather effects from ALL rows. It basically wipes the slate clean by nullifying any weather effects used against you for a single round. Furthermore, this leader card isn't as "telling" as others. If your opponent is using the Foltest - Siege Commander card which doubles the strength of all your siege units when used for example, then from the start of the game you know you'll have to face that move from your opponent at some point. Basically meaning save a scorch card or two until the round your opponent uses the Siege Master ability and nuke all their siege to nothing. It's just too easy to prepare for. With the Foltest Lord Commander of the North ability your opponent is pretty much helpless against the fact that at some point during the game you're clearing all weather effects from the board. Period. There's just not nearly as much counter-strategy that can be made against this ability as others. Don't get me wrong, many other leaders cards have great abilities such as the Eredin Destroyer of Worlds leader card that lets you return a card to your hand from your discard pile (much less helpful for a faction that doesn't rely heavily on spy cards than it would be otherwise but that's beside the point), but I still feel these leader cards pale in comparison.
Finally, I feel unit wise the NR set is simply more well rounded unit wise. Both Monsters and Scoia'tael rely heavily on muster cards but this can backfire when the cards the rally pulls are all straight from your hand. It's pretty frustrating to start a hand with 3 arachas (Monsters) so that as soon as you lay one down your 10-card hand is now down to 7 on the first play... for a mere 20ish power points. Of course it's great when you start with only one of the specific muster card types so that you play it, your 10-card hand goes to 9, and you're dropping 20ish power points on the board with a single card. Here though we are back to both relying on chance to go in your favor which will frustrate you as often as it'll please you. All-in-all it's just harder to strategize with such variability as opposed to a more reliable NR deck. Niilfgaard, unit wise, suffers from a lack of versatility compared to the NR units in that it has higher point spy cards than the NR spies (Nilf Spies Steffan Str: 9, Shil Str: 7, Vattier Str: 4 (Avg Str: 6.33) compared to NR Spies Stennis Str: 5, Dijkstra Str: 4, Thaler Str: 1 (Avg Str: 3.33) - Recall low strength spy cards are preferable to high str spy cards as when played spy cards add their unit str to the oponnent's total str).
NR units are also less susceptible to being scorch targets. For example, 2 Nilf. Black Infantry Archers give 20 ranged attack power but as soon as you lay one down you're drop a sinlge card worth 10 on the board which seems a tasty target for scorch. Comparably, you drop a single NR Crinfrid Reavers Dragon Hunter Archer down and it's only worth 5... not a godo scorch target as your opponent would more than likely scorch one of their own higher power cards than a measly 5 power card. Yet due to the tight bond perk, you drop down your second Crinfrid Reavers Dragon Hunter at an opportune time and both cards double their power to 10 and you have 20 ranged attack power from 2 cards, same as the 2 Nilf Black Infantry archers, but weren't an obvious scorch target from the get-go.
In fact NR, has a solid "Tight-Bond" special ability across all three unit types unlike the other factions. Examples:
Melee: Blue Stripes Comamndo (Str: 4) - can have 3 in deck. With all three played = 36 power (72 if you throw in a commando's horn for an avg power of 18 per card between the four cards (3 Commandos and 1 horn))
Ranged: Crinfrid Reavers Dragon Hunter (Str: 5) - can have 3 in deck. With all three played = 45 power (90 if you throw in a commando's horn for an avg power of 22.5 per card between the four cards (3 Reavers and 1 horn))
Siege: Catapult (Str: 8 - can have 2 in deck. With both played = 32 power (64 if you throw in a commando's horn for an avg power of 21.3 per card between the three cards (2 catapults and 1 horn)
With this kind of versatility you have the opportunity to drop huge power numbers on any unit type row in practically any game you come across. By using all of your spy cards and max decoy cards (3) when your opponent drops his own spy cards to allow you to play his own spy cards against him/her, you can spend the early game stockpiling these "Tight-Bond" ability cards with your opponent have virtually no clue which type of unit you're going to focus on each round. You can always drop a Str: 5 Crinfrid Reavers Archer in one match, and if ranged isn't going to be a good focus for you that round just let him die and resurrect him next round with a medic card allowing you a versatile way of playing your maximum str cards in a fashion, as said before, that doesn't make them immediate scorch targets..
/End Encyclopedia on Gwent Strategy lol