The usual ones that don't take a ton of processor power:
MAME
Daphne
Zinc (if your processor is too slow to run psx based arcade games like tekken in MAME)
Model 2 Emulator for Sega Model 2 games
New School..requires a bit more horsepower
There are MAME builds out there that run more recent Cave 3D shooters.
Supermodel for Sega model 3 games
Demul for NAOMI, Atomiswave, & Gaelco 3D games.....Sega Hikaru is in early stages
Makaron for NAOMI games that my processor won't run full speed in Demul
Taito Type X, X2 - not an emulator, but arcade games that ran on embedded xp and have been cracked to run on standard windows (games as recent as 2011, maybe 2012 IIRC)
Those are my core. Using some tricks from the Mala subforum, they all appear together in my main arcade gamelist regardless of emulator.
Oddballs not used as often:
Dice emulates old discrete games like pong.
Daphne Singe which makes the American Laser shooting shooting games more like the original arcade versions. It also converts Hologram Time Traveler into the arcade version.
Viva Nonno for the Ridge Racer series
There is a Triforce version of Dolphin emulator that will run Mario Kart GP2.
That's all I can think of for now. I'm probably leaving some out.
EDIT: refer to system 16 to see what games were on the Sega Model 2, 3, Naomi hardware.
http://www.system16.com/museum.php?id=1Taito Type X
http://www.system16.com/museum.php?id=8Atomiswave
http://www.system16.com/hardware.php?id=812I play about 25% MAME, 50% new school emulators, & 25% steam games on my cab (Mortal Kombat Komplete, Street Fighter X Tekken, etc)
The NAOMI shmups like Ikaruga and Border Down sucked me into building a new school cab.