When I first played the game, I chose Emboar because I'm a sucker for fire starters. However, my game was stolen, many many years later, a friend knew I got a 2DS and was getting back into Pokemon so he gave me his Pokemon Black 2 as a gift. So thanks to him, I decided to switch things up and start with Snivy. I have no regrets, there are other fighting and fire types you can use including Lucario and Darmanitan, but it does suck that despite Serperior's dope design, a lot of pokemon have a resistance against grass types.
I imported Black because I knew I wanted White in English so my first playthrough was using some stuff that I knew I wasn't going to use in my normal first ENG playthrough but also wanted to use. So for JPN Black I chose Oshawott and in White I chose Snivy. In White 2 I chose Tepig just to round it out but I was reluctant with that pick; I really am not a fan of any of the Generation V starters and they haven't grown on me other the years.
I love all 3 of the Unova starters and they are my second favorite starter trio ever (Johto trio being my favorite). That means that I chose each one at least once but my favorite is Serperior even though that wasn't the case when the games first came out. I think that you should know that Gen 5 is my favorite generation before I start to list what starter I picked for every Gen 5 playthrough I did because there are a lot of playthroughs to go through.
My first Gen 5 playthrough was in Pokemon Black and I chose Oshawott since it was my favorite back then but I remember being a little disappointed with Samurott's design since it's sprite looked weird to me because it looked like it was floating or standing on it's toes/claws and Samurott's pose was also weird to me. I grew to like it more over the years but it is still my least favorite final stage design of the Unova starters, especially when I compare it to Oshawott and Dewott since I love their designs sooo much.
Because of this I decided to pick a Snivy when I got Black 2 and that playthrough and team were so amazing that I still consider it one of the best and most fun playthroughs I ever did. Because of the great experiences during this playthrough Snivy line became my favorite Unova starter and my second favorite starter of all time.
Then a little later I got Pokemon White 2 and chose Oshawott again since I started to warm up to Samurott's design around that time but I also traded myself a Snivy Egg from my Black 2 so I had both Serperior and Samurott on my team.
After this the Gen 6 came out and I got more into competitive Pokemon scene and then even later when Hidden Ability Unova starters were released I started to like Emboar a lot because I had a lot of fun with Reckless in lower tiers on Pokemon Showdown. Because of this I decided to reset my White 2 and use Tepig as my starter, I did just that and had a ton of fun with Emboar and it became my second favorite Unova starter.
A little later I wanted to replay White 2 again and I chose Snivy again. Then even later I was able to buy a German White so I played through it and I chose Oshawott again since I didn't use it in a long time. And now my last Gen 5 playthrough was early last year after I got White (in English this time) and I chose Snivy since I wanted to do a Grass Monotype challenge.
I also plan to do a Tepig playthrough sometime this year since it's the starter that I used the least despite the fact that it's actually my second favorite starter line from this region.
I've replayed Black and White a crap ton of times, I usually pick Tepig, because it's hard to find a good fire type early game and I refuse to use any of those stupid monkeys. I also really like Snivey, so it's the runner up.