I am just getting back in after 20 years so I can really only give opinions on loco's and rolling stock as they run around my test track (Bachman EZ track 11" radius) till I am ready to start building. I work part time at a shop that specializes in RC cars and planes so most of my purchases have been what I have available to me and ebay.
Loco's (mostly Steam)
Bachmann:
Spectrum 2-8-0 Consolidation - Best running loco I own, has pulled 35 cars on level track, easily pulls 25-30. Geared to crawl along the track. Very smooth, quiet and detailed.
Spectrum 2-6-6-2 H5 - Beautiful running, no traction tires so about 20 cars is the max it can pull on a grade. (Note: The one I bought was a lemon, contacted Bachmann and it was taken care of promptly, excellent service.) I have a friend that also bought one, his runs flawless out of the box. Make sure to break it in, they run even better after about 20-30 minutes of running. Geared to crawl along the track. Not as smooth or quiet as the 2-8-0 but very impressive for the amount of moving parts and detail that it has.
Spectrum 4-8-2 Lt. Mountain - Similar drive train to the Consolidation. Very smooth and quiet running, not nearly the pulling power of the consolidation. Has traction tires but they are on the second axle and don't get the full weight of the loco. Excellent detail.
Spectrum Dash8 - Came with my starter set, smooth, diode issolated Fwd and Rev lighting, quiet, decent pulling power (about 25 cars). I'm not a diesel fan so this one doesn't get run anymore.
0-6-0 Switcher - Cheap little switcher, pickups in tender, runs OK, nothing spectacular but they are very inexpensive.
Model Power:
2-8-2 USRA Mikado - Noisier than the Spectrums and geared a little faster. Metal boiler, not as detailed (molded in handrails on the boiler). Pulling power is adequate without traction tires. MDP offers a traction tire update for the engine that makes a huge difference in pulling power. I have performed the traction tire conversion on this engine, it is by no means a simple operation. Be carefull not to pinch wiring when re-assembling. With traction tires, I don't have enough track on my test track to load it down with cars. The test loop is roughly 4'x2' and it will pull a train the the entire loop (about 40 cars). Sometimes tempermental on switches.
4-6-2 USRA Pacific - Noisier than the Spectrums and geared a little faster. Metal boiler, not as detailed (molded in handrails on the boiler). Pulling power is adequate without traction tires. MDP offers a traction tire update for the engine that makes a huge difference in pulling power. Same boiler and drive as the Mikado
LifeLike:
E8/9 Powered A/B set (latest version)- Smooth running, strong, quiet, excellent detail, strongly suggest converting to MT couplers, at least between the A and B unit. They have body mounted coupler and because of the long overhang, tight corners tend to put a stress on the stock rapido couplers and they uncouple. Especially going from left to right without a transition. I don't have enough track and cars to bog these two loco's down when paired up. running individually, they can pull roughly 30 cars each.
Older loco's
ConCor PA/PB Powered Set - Noisy, great pulling power, geared to fast. Very reliable engines. I had a set of these 20 years ago with my last train, picked up another set off ebay for nostalgia. (sp?)
Atlas/Rivarossi 4-6-2 Pacific - Detail appropriate to the age of the loco. Minimal pulling power, will pull a set of 6 heavyweight passenger cars around comfortably which is all I expect of it so I am satisfied. Geared reasonably low for its age, smooth runner. One rail pick up on tender, one rail pick up on loco so can have intermittant running issues if track or loco is not kept clean.
Rolling Stock:
Atlas - Newer cars with accumate couplers and trucks my favorite. Great detail, price and reliability.
MTL - Best detail and most accuate cars available, often very pricey and limited availability.
Intermountain - I only have one 40' box and the detail is great but the trucks and couplers are junk. Not worth the cost in my opinion unless you can't find the car by any other manufacture. I wouldn't have bought if it wasn't for my shop discount. Also movable doors on the car fall off at random.
MDC Roundhouse/Athern - The cars are nicely detailed but too light. They tend to need weight added to run reliably. MDC knuckle coupler only works reliably with other MDC equiped cars, they will uncouple from MT couplers with enough load or uneven track. Suggest replacing trucks on the cars. Horizon Hobby recently bought both MDC and Athern and are reorganizing the lines. It appears most of the RTR MDC N scale is going to now be produced under the Athern name and they may come with better trucks this time around.
Bachmann - The cars are decent in detail, very reliable, weighted right but are not easy to convert to Atlas trucks w/accumate couplers. Need to open up the hole in the truck to fit over the molded pin on the car. Trucks held on with screw. Price is usually right.
Lifelike/Industrial - Cheap cars, detail is acceptable for background filler and most are ok runners. Replace the trucks and or wheelsets for better reliability.
That is about all I have experience with right now. Hope it helps.