A few of my better days during the past year or two:
6/13/04: Central/Northern MI... This day had a cold front situated across eastern WI, with a warm front stetching back to the east across central/northern MI. An isolated supercell exploded to the north of Grand Rapids, and quickly became a prolific tornadic supercell that persisted for about 5 hours. Obviously, I have a warm front to thank for that day.
6/9/05: Northwest KS... This was probably one of my coolest days ever chasing. This day had dryline situated from extreme southwest NE then southward into the TX panhandle. In northwest KS, a outflow boundary intersected the dryline near Hill City. Deep moist convection initiated along the extreme northern end of the dryline across southwest NE and into northwest KS, and quickly moved away from the boundary and died. To the south, a supercell exploded right on the OFB/DL intersection... And this would become one of the most famous supercells of the year. Supercells also formed south on the dryline, as well, from southwest KS into the TX panhandle -- also producing some tornadoes. This was a dryline/OFB day.
6/12/05: Northwest TX... This was probably the best chasing day of the year, and probably my chasing so far. This was another dryline/OFB day... Cells continued to pop along the boundaries all afternoon, one becoming a SVR-warned supercell and then quickly dying. Then, by mid-afternoon... More convection initiated... In the form of a cluster - but the storms quickly organized into two incredible supercells. The northern one was the first to go TOR-warned, but didn't look worth my time... So I headed south towards the southern (well, middle one) supercell and by the time we all got there (chased with Kurt H, JR Hehnly and Dan Robinson that day) it produced the first tornado - and another half dozen or so followed.
5/12/05: Northwest TX... Believe it or not, I actually have a cold front to thank! A cold front plowed through the TX panhandle during the afternoon, initiating a long-lived supercell that formed in the Plainview area. By the time we got south towards the now developing supercellular complex, all tornado warnings was withdrawn. Then, we got to the storm near South Plains... And it immediately dropped a beautiful white cone (then barrell) tornado. Then, it followed with a large wedge and a brief trunk. We were lucky to find ourselves with windshields after we chased that storm.
5/13/05: Northwest TX (lol, I am so biased now)... A large group of chasers (including myself, Kurt H, Amos M, Jason M, Tony L and about a dozen others) woke up in the day's target! Talk about a treat... A dryline/cold front/OFB intersection was in place at 18z... And a pronounced mesolow was over northern Floyd county. The OFB became more diffuse and lifted northward into the Childress area. This would be combined with a very moist and unstable boundary layer... With a SFC moist axis containing 60-65 Td's and sbCAPE was a respectable 2500 j/kg. Well formed Tcu persisted across the Childress area, which we had the pleasure to watch from a resteraunt parking lot there in Childress. A storm just to the south of Childress exploded, and about 2 scans later, it was SVR-warned and was showing supercellular traits. This day pretty much was thanks to the OFB and dryline.
In the upper midwest, I am pretty biased towards warm fronts and OFBs. In the plains, I am pretty biased towards drylines and OFBs... Particularly in the TX panhandle.