title says it all

The first champion I bought with RP was Ryze when I got the 400RP from the Summoner's Rift Tutorial. The first champion I bought with IP was Annie I believe.

Ashe

Warwick
...
Still play on regular basis.

Taric I think. Still regret that decision

Blitzcrank with IP, Nautilus with RP

Ezreal, I rushed for the 6300 IP when I first started playing and got him within the first week of me playing LoL. I still main him. First champ I got with RP was Darius, well, because he's Darius.

Fiddlesticks was my first IP champ. I liked his Drain, because that meant I could stay in lane longer and not have to recall (I hated recalling when I started out, haha). I stuck with AP mids (mainly Heimerdinger, my second IP purchase) in my earlier days before settling on support as my main position of choice. Oh, I should note that I played bot Fiddlesticks in those days (I didn't feel comfortable enough to lane alone, haha), and didn't even dare to try jungle until I was forced to as Nunu when playing with a couple of friends. It didn't go so bad, but it was definitely a learning experience!

I believe Elise was the first champion I bought with RP, but she wasn't what I had expected and promptly refunded the RP to put toward Zyra (I think - it may have been Xerath).

-Chaz

Despite the fact that I played Zilean almost 24/7 when he was free during my first week, my first champ I bought with IP was Lux. As for RP, I bought Ezreal since barely anyone used him at the time.

Lee Sin with RP I believe, Ryze with IP