Top lists of things are always subjective, so I'm not going to make any wild claims here. Instead, here are some Mudrunner mods that I've enjoyed using, and why.
Vehicles
There are plenty of mods that provide overpowered offroad monsters, but blasting effortlessly through mud and scaling every slope with ease isn't what Mudrunner is about.
These vehicles all have some limitation that makes driving them through Mudrunner's sticky expanses both a challenge and a joy.
Lada 4x4
For me there's nothing more satisfying in Mudrunner than getting an underpowered vehicle through a particularly boggy map. The Lada 4x4 fits this requirement perfectly, comes in a range of colours and has a huge amount of garage customisations from roof boxes to door handles. A lot of effort has gone into the model - you can even open all the doors.
GAZ-SiVER
The GAZ-SiVER is a heavy truck with big wheels and high ground clearance but is also surprisingly top heavy and prone to rolling on uneven surfaces. It'll take on most terrain but requires care to use properly.
btr80
The btr80 is overpowered in a satisfyingly believable way. It's too large to fit through small gaps, realistically heavy and impossible to sink. It also features a rotating turret and opening window flaps, which is an entirely useless but nice detail. It's good for surviving maps with unfairly deep water.
Toyota Hilux
There are plenty of Toyota Hilux mods available for Mudrunner. They vary in realism and satisfaction, but all of them are good off road.
KAMAZ-65228
The KAMAZ-65228 is a big and heavy truck with a lot of power. It won't manouver well in tight spaces and is difficult to right if you roll it, but with the right combination of trailers it can carry 8 log points in one trip, a task that it can handle with relative ease.
Уa3 SwaP
The Уa3 SwaP looks like it's based on one of the default trucks in the game but feels different to drive. A good solid jeep, it's not too powerful and works well on most maps.
JEEP-1942
With no current addons and a very low profile the JEEP-1942 might not immediately appeal, but it offers a similar experience to the game's A-969 without being ridiculously underweight. I enjoyed driving it from the driver's perspective.
Maps
I mostly play MudRunner in multiplayer and I find that co-operative log hauling doesn't hold as much appeal as racing to unlock watchpoints, scouring the landscape for the points to unlock a garage, or simply exploring with friends. That said, there are plenty of maps that simply provide new and interesting things to see, often through custom built scenery. Here are some that I have enjoyed.
Tynda-10 Timpton
Tynda-10 Timpton provides a well thought out garage unlocking challenge. The only garage points are with a truck already stuck in a river with almost no fuel and badly in need of repair, so players must co-operate to refuel, repair and rescue the truck, before making their way along a road mostly washed away by a fast flowing river - towing is recommended. Done with the default trucks, this is an entertainingly difficult challenge.
Back Woods 2 - Aftermath
Back Woods 2 is set in a village that has recently suffered a catastrophic flood. There are no watchpoints, but a submerged road to follow carefully through a large lake forms the main objective. Garage points must be retrieved from a fast flowing river passing over a road.
Beautiful
Initially I thought Beautiful was well put together if disappointingly small, but as a logging map it turned out to be very well designed and laid out, featuring some well thought through routes and precariously adverse cambered roads. It also looks pretty good.
Reckoning
Sometimes a map is really well made. Reckoning is one of those maps. It's not stupidly challenging, it's not overly ambitious, it's just well put together. Lots of interesting roads, some clever river crossings, and a good bit of challenge to find the necessary items to unlock garages. Recommend you play with the default vehicles.
Полюс Холода
There are a few snowy maps available but this is the best one I've seen so far. The snow is thick and sounds realistic under your tyres and the ice allows for satisfactory skids and sliding. There's also a load of custom scenery including snowmen and ice sculptures and the whole thing feels very wintry.
I encountered some entertaining bugs when exploring it using the GaZ 67RS Snegovik where my truck would be suddenly catapulted high into the air when driving over ice or thrown halfway across the map into the trees. Seems to be fine with the standard Mudrunner trucks, but worth experiencing for the comedy.
MadSummer
MadSummer is another modest map with sprawling trails, interesting roads, and just when it all seemed really straightforward I found myself driving an articulated lorry into a swamp. It also features two garages to unlock and plenty of countryside and two delapidated towns to explore.
Belarusian Polesie
It's not a very detailed map, but Belarusian Polesie offers a series of islands connected by submerged roads through some treacherously deep water. Exploring and finding a way through offers a good challenge. The map also starts with a long and lopsided bridge over a deep river, featuring gaps that you can lose a wheel in, leading to a garage trailer at the far end. I didn't manage to retrieve it, but I had fun trying.
Yuma trails
There are a few desert maps available for Mudrunner and they provide a somewhat different experience to the base game, with scrambling over dry rock often entirely replacing wallowing in mud. Yuma Trails takes this to new heights with multi-level caves to explore, hidden entrances and even a small scrambling course marked out with cones. At times I felt like I was playing an entirely different game. Take a good truck - you'll need it.
The Mountain
Another style of map is the 'single route' design, where the route is basically linear with little deviation possible. Done well, it can be a delight - done badly, a chore. The Mountain is one of the better ones, offering challenging climbing and descending, a good variety of scenery, and an interesting 'save' mechanism, where the same truck appears at infrequent points along the route, offering a backup if you've managed to destroy your current ride.
Honourary mentions
This BMW X6M is realistically modelled in that it's almost entirely useless off road. But if you want to feel like you've climbed a mountain, try using it on an unfairly muddy map like level_klein_4pil_8pil. Getting to any watchpoint is a considerable achievement.
The Wertohod EC-635 is a entirely ridiculous vehicle, apparently built from the remains of a flatbed truck and a helicopter. It has lots of options for adjusting the wheels and turns by hinging in the middle, making it prone to getting easily tangled in trees.
And if you really want something totally over the top and likely unsuitable for most situations, the Komatsu HD985 has your needs covered.