What brands have you had the best experience with for:

F40T12 preheat, magnetic:
F40T12 rapid start, magnetic:
F96T12 slimline, magnetic:
F15-F20T12 preheat, magnetic:
F32T8 electronic:
T5 HE/HO electronic:
Other types (i.e. circline, T12HO, small T5):

I have had great success with all preheat magnetic F40T12 ballasts.  I have used the most of Valmont/Power Lighting Products 8G1000W and Universal 205-TC-P for my replacements and conversions and had no problems with either.
I like just about all full power (Pre 34/35 watt lamps) F40T12 magnetic rapid start ones also, but have had the most experience with the Universal, Advance and General Electrics. Full power Universal F96T12 ballasts (before the 60 watt lamps) are the best I have seen in the slimline department, in my opinion.

Ditto what John said about mag ballasts! I have had problems with Universal brand rapid start F40T12 magnetic ballasts (mainly noise and early failure). Advance and old GEs seem to be the best in my opinion. Newer GE mag ballasts are junk. They can't handle any heat, fail quickly, and are noisy. Lately, I've stayed with Advance.

I'm just beginning to test electronic F40T12 ballasts (thanks, Uncle Sam & EPA). I just ordered a Howard and an Advance for comparison.

I try to avoid T8/T5 ballasts so I can't say much about them.

-Nelson

I have quite bad experience with magnetic fluorescent ballasts inside plastic fixtures - losses and consequently the operating temperature are simply too high to damage the fixture.
Cheap electronic ballasts like to fail soon, but usually it is not as difficult to repair. I usually review their design and very frequently carry on some very simple and inexpensive mod. And it seems to work - no such modified ballast failed again yet. What i do not understand, why such simple and inexpensive precautions are not present already in the original design - many of them cost nothing, only different connection of already present components or even their mechanical placing (e.g. the electrolytic capacitor in the CFL not soldered directly onto the board, but kept on longer wires, so it would be placed in the much colder socket area)...

@nogden, I have had great success out of a Howard Industries F40T12 electronic rapid start ballast, and I have ABUSED it frying EOL lamps!

For F40T12 magnetic rapid-start:
 in general they've been fine for me.

For electronic F32T8:
'AAA' brand = Pure Junk
'Power Lighting' brand = pretty crappy
'MagneTek' built GE's - the one i had failed on the first EOL lamp it saw = so i'm gonna say junk
(can't say on Advance or GE Ultramax or Philips...only time will tell)

For electronic T5:
Sylvania: (tested only) haven't had enough time to say

I have all electromagnetic ballasts. Here are the ones I like the most.

T12
My set of four vintage Universal Thermomatic X ballasts (full length wires)
Advance "Powrkut" F40T12 ballast (hybrid magnetic/electronic)
Vintage Robertson Transformer Co. HPF 14/15/20W ballast
Metalux preheat strip with advance choke (also fires up F17T8 lamps perfectly)

T8
Universal Magnetic RS 2-lamp ballast
Lumikon brand magnetic RS 2-lamp ballast

I use F32T8 electronic if its left on for a LONG time, due to best energy savings, and less heat. Such as for my grow lamps. I use rapid start though for frequent on/off switching though, for cabinets, and my desk. They save energy more for that and give a longer bulb life.

Electronic ballasts Philips or osram all magnetic is good! 

Electronic Ballasts:  from Philips, Osram ( Siemens ), VS and Tridonic

Magnetic Ballasts: Potted Fluorescent Philips RS ballasts, various Knobel Ennenda ( Now Tridonic ) ballasts and Leuenberger of course...

I live in the 230V area, so i have two options:
- Series choke (magnetic): No issue with any ballast, but had overheat issues when the fixture was not able to handle the heat.

- Electronic: No problems with OSRAM, however they seems to be not protected against condensing humidity, so need really sealed (gas-tight) fixture when exposed to humid air.
Then i use my own designs (because i wanted dimming working with existing wiring and none such product is on the market); no PFC, overvoltage protected by VDR, assembled PCB "soaked" in moisture protection coat, no failure yet (operated even with the condensing humidity)