Apple's upcoming iPhone 5 may be the biggest competitor in the smartphone market for Samsung Galaxy S2. But Galaxy S2 still faces a lot of threats from others who are bringing in game-changing devices into the ongoing smartphone battle.

Korea-based Samsung is facing considerable competition from other Android smartphone makers. Its long-awaited flagship Galaxy S2 will soon arrive in the U.S., but some challengers will try their best to spoil Samsung's party.

Galaxy S2 runs on Android 2.3 Gingerbread OS and sports Samsung's TouchWiz UI. The device sports an 8 MP rear camera with flash and can record HD video in 1080p, offering the richest video content on a smartphone. The phone also has the front-facing 2 MP camera for video chat.

Samsung Epic 4G Touch, carried by Sprint, comes with a 129.5 x 68.6 x 9.6 mm dimension and weighs 129 g, while the T-Mobile version, popularly known as Hercules, has 131 x 70 x 9 mm dimension. The AT&T version, which retains 4.3-inch display, is expected to feature 125.3 x 66.1 x 8.5 mm and weighs 116 g.

Both Hercules and Sprint Epic Touch 4G have 4.52-inch display screens. The AT&T version, meanwhile, retains the 4.3-inch screen of the original Galaxy S2 that has made a name worldwide. It is powered by a dual-core 1.2 GHz ARM Cortex-A9 Mali-400MP GPU Orion chipset. The phone comes in 16/32GB internal memory configuration with 1GB RAM.

However, here is the list of the 5 most challenging smartphones that packs enough firepower to see off Samsung's dominance in the smartphone domain.

Motorola Droid Bionic: Droid Bionic is a smartphone that has made quite a name for itself with its release. The device runs on Android 2.3.4 or Gingerbread juice and looks similar to Droid X2, which is another dual core smartphone.

The smartphone sports a 4.3-inch, 540 x 960 pixels, TFT capacitive touchscreen and is powered by a dual-core 1GHz ARM Cortex-A9 processor, ULP GeForce GPU, Tegra 2 AP20H chipset. The device also sports an 8 MP, 3264x2448 pixels, autofocus, dual-LED flash camera with video recording at 1080p. A decent VGA camera has also been provided for video chat.

Other notable features include Wi-Fi 802.11 b/g/n, DLNA, Wi-Fi hotspot, proximity sensor for auto turn-off and LTE. The device also comes in with an HDMI port, Google Search, Maps, Gmail, YouTube, Google Talk and Adobe Flash 10.1.

HTC Holiday: HTC Holiday, which is rumored to boast a 4.5-inch display, is supposedly the first of a new line of Android-based smartphones with 4.5-inch displays.

Holiday is rumored to have 1.2 GHz dual-core Qualcomm Snapdragon processor with 1GB RAM, Android 2.3 Gingerbread OS, 4.5-inch qHD display with 960x540 screen resolution, 8 MP dual-LED flash rear-facing camera with 1080p video playback and a 1.3 MP camera for video chat. Other sources have also talked about HSDPA up to 21 Mbps, HSUPA up to 5.76 Mbps, Wi-Fi 802.11 b/g/n DLNA support, advanced Bluetooth version 4.0 with A2DP and EDR, and microUSB version 2.0.

HTC Holiday is expected around mid-September with a price-tag of $199 to $249 at AT&T.

Google Nexus Prime: If rumors are to be believed, the Nexus Prime will most likely be the first smartphone to run on Android 4.0 Ice Cream Sandwich and will arrive in October, according to Korean Web site Electronic Times.

The Nexus Prime, tagged absolute beast among smartphones, is expected to boast a dual-core Snapdragon processor that will include Krait chips from Qualcomm, and a 1GB RAM. It is not yet known if the processor will be of either 1.2GHz or 1.5GHz.

The Krait-based Snapdragon processor will have the ability to run at an amazing speed of 2.5 GHz per core at the least, which will make it more than 100 times faster than a normal ARM-based CPU.

Previously, Tech site BGR had presented a sneak peek into the device's features, which they called Nexus 4G. According to that, the device would feature a next-generation dual-core 1.2GHz or 1.5GHz CPU as well as an OMAP 4460 or a, ultra low-power 28nm Krait-based Snapdragon.

They added that the device will feature a 720p HD monster-sized display, a 4G LTE radio, 1GB of RAM, 1080p HD video capture and playback, a 1 MP front-facing camera, and a 5 MP rear camera.

HTC Vigor: Vigor is rumoured to hit Verizon's 4G LTE network. It is also said to sport a 1.5GHz dual-core processor. The smartphone, said to be the replacement for the HTC Thunderbolt, is also rumored to arrive with a 4.3-inch display with 1280x720 screen resolution.

Vigor, as per sources, is expected to sport an 8 MP camera and run on Gingerbread juice. It is also expected to offer 1GB RAM, 16GB internal storage and inferred to incorporate Beats Electronics Technology which, probably, is best known for the high-performance line of headphones backed by hip-hop artist /producer Dr. Dre.

The device's leaked pictures were recently posted on Droid Life.

Motorola Photon 4G: Motorola Photon 4G has 4.3-inch qHD capacitive touchscreen (960x540 pixel resolution), multi-touch input method, accelerometer sensor for UI auto-rotate, proximity sensor for auto turn-off and touch-sensitive controls.

Also included is a 16GB storage (up to 32GB with microSD support), 1GB RAM, 2GB ROM, Wi-Fi, Bluetooth, 8-megapixel camera (with dual-LED flash, autofocus and geo-tagging features), secondary VGA camera for video chat, Android 2.3 OS (Gingerbread), 1 GHz NVIDIA Tegra 2 AP20H dual-core processor and HDMI port.

The Sprint Photon 4G is a world phone which means the users have international GSM roaming so that they can access their email, calendar and contacts from anywhere. The device could also attract corporate users with its enterprise-tailored security.

The device is Sprint's seventh Sprint ID-capable handset. The Sprint ID delivers number of applications, shortcuts, widgets, ringtones and wallpapers. The pack provides free-to-download stuff under its Everything Data plan, and users are allowed to download five packs at a time.

Photon 4G is priced at $199 on Sprint with a two-year contract. Amazon Wireless, however, offers the phone for $99.99 for new Sprint customers.

The device's reviews have mostly been positive.