Zebrastrika- While certainly not the best, this speedster did help out in crucial moments. I always liked the design, but I already had Ampharos when I played Black 2 (and another electric type or two during Blaze Black 2).
Mamoswine- Since I never played 4th gen, and then fully hopped back into Pokemon with 5th gen last year. I never really liked Poliswine in Gen II, but I did see a few episodes of the 4th gen anime with Dawn's Mamoswine. I tried it out and found out it's a complete monster.....it's the main reason I beat Ditto-rseus without spending an item like the focus sash.
Goomy/Goodra- Reborn is the first time I have ever tried breeding in a Pokemon game (mainly because of the IV/EV reader and the speed up button), and Goomy ended up being my first breeding project. I still have yet to use one in battle (either in-game or against a player), but I grew attached to the slimy dragon through all the bike riding.
Pachurisu- While I didn't use the event Pachurisu during my first (only) run of the game, it has proven very helpful so far in my run of Reborn Hardcore (kind of stalled at the second gym due to temporary repeat-fatigue). It can sure take a beating.
Sort of mentions: Espurr/Meostick, Pyroar, Panguro, and Greninja--- One of the main reasons I started Reborn in the first place was it's inclusion of almost all 6th gen mons so far, since I will probably never get to play the canon 6/7th generation games. All of these caught my eye and I knew I wanted them for my in-game team. Technically doesn't count, since I already loved something about them, but they deserve a mention.