Before switching to MyBB, what other forum script were you using ?.
If MyBB was/is your first forum and you're still using, just say MyBB and tell us what version number you started with.
I have used SMF and phpBB before. SMF was pretty nice and easy to use, but as far as modifying core files, themes, and templates, it can be a pain. MyBB's plugin system and template system are hard to really beat as far as free software is concerned.
I've used Invision (it was still free then) and phpBB. When it became clear that phpBB 3.0 wouldn't have a plugin system I started to switch my forums to MyBB in 2006. So my first MyBB version was 1.2.1.
Oh I remember using punBB as my first forum IIRC. That was some years ago.
I've used punBB and phpBB for the first months of starting out...coming to MyBB was the best thing I could do.
I've used Phorum, a few others I can't remember, then SMF, vB 3 and vB 4, then PhpBB ,and now MyBB.
I used FreeForums.org, which used phpBB.
IPB, back when it was free.
My first forum was Proboards. Self installed wise was phpBB 2.0.6. Not long after that I was looking at MyBB and OpenBB. Ended up doing a fresh install (no converter back then) of MyBB Release Candidate 4.